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Discussion Starter · #1 · (Edited)
Jetting 101

This picture below (Small one Bottom Left) shows a good plug with extended use or a lot miles on it with mixture, heat range and ignition timing appear to be set correctly. New plugs used for plug chops will not or should not show this much color or deposits on them. Although new plugs and plug chops will show color and deposits on the base ring and deep inside the plug where the ceramic attaches to the plug body. A new plug or cleaned plug used in plug chops will appear with much less color and deposits but still give you an idea of where your jetting is set. After several hundred miles your plug will probably start to look more like this plug picture below. I've also attached a picture of a new plug with a single plug chop run on it. ("Slightly Rich") Probably what you should be looking for ideal power. Beware, your computer monitor brightness settings, can distort the colors and brightness of what you see. Read and understand the text carefully.

Jetting the starting point
Fuel delivery is controlled by throttle position, not engine RPM or speed. Therefore it is important to adjust your jetting based on the position of the throttle: Closed ( mostly PMS setting), less than 1/4 open(mostly pilot and some PMS & a little Needle), 1/2 open(mostly needle height/diameter) 3/4 open(mostly needle height/diameter and some main), full open (mostly main jet). Before you begin check the following. Is your air filter clean? Is the spark arrestor screen clean? These two items can affect your jetting results. Performing plug chops at various amount of throttle openings, 1/4, 1/2, 3/4 & Full open then reading the plug that was cleaned before each plug chop is the key to jetting correctly. Depending upon what your plug is telling you at those different amount of throttle openings and jetting in the direction the plug dictates will get you dialed in.

Re-jetting also requires having a good selection of jets on hand. You dealer or your local HD motorcycle store may have have pilot jets that fit, stock chances are your pilot is a #40, I got a 42.5 and a 45 from a local Harley Davidson speed shop that carried them in stock for my 650H1. Your ATV dealer should be able to order them also. Main jets can be bought in a kit form from places like Dynojet and individually from many places online such as CountryCat, Speedwerx or through your dealer. If going into a HD speed shop to get pilot jets, take your old one in to insure the replacement matches what you have. I'm not certain if all AC carbs use the same type of pilot so beware buying HD pilot jets. Do not mix Dynojets jets up with Keihin jets, the numbering scheme is different and fuel flow characteristics are different.

Warm the engine up good, set idle speed then adjust PMS screw by opening up at least 3 turns and slowing closing until maximum engine RPM is reached (go by sound if no tachometer) then open approximately 1/4-1/3 turn further open then the maximum RPM point. Adjust RPM and repeat if necessary. Hopefully the resulting number of turns open on the PMS screw will be between 1-1/4 to 2-3/4 turns open at this point. If the final setting is closed more then 1 turns open then a smaller pilot jet is probably needed. If further open then 2-3/4 turns then a larger pilot jet (slow jet) is probably needed. (NO TWO CARBS WILL BE THE SAME) For twins, set one of the cylinders to around 2 1/2-3 turns open as you set the other. Once the first cylinder is set you can then set the other. Leave the first PMS setting to your optimum setting once it's found while you adjust the second. After setting the second, retest the first again though. See section for Carburetor balance too.

To set the jetting.
After the number of turns open on the PMS screw is set. Open the throttle just far enough to move you at a steady slow speed for about a mile, this should be about 1/4 open or one fourth of the total movement between idle and wide open on the throttle thumb lever. Do a plug chop as described below and read your plug. At this point you can adjust pilot jet (slow jet) for correct plug color but you'll probably need to reset PMS turns open again and recheck. The engine must be under load but not lugging. A CV carburetor will self lean out when the engine is under light loads. This is normal and desirable for best mileage but it does make plug reading a little more difficult when setting jets for best performance.

Next, Hold a WOT (wide open throttle) for a 1/2 mile. Plug chop and read the plug. Finally set needle height with a plug chop and read. Do a run at 1/2 open throttle raise or lower the needle height to obtain proper plug colors. To raise a stock Arctic Cat needle, You can use #4 machine screw washers. Each washer is about 0.020" thick and if needed you can stack several of them together to raise the needle. This method will set all three fuel circuits above idle for your carburetor. If you raise or lower the needle then you'll need to recheck WOT too. Ideally, if you change any jetting circuit up or down, retest all circuits over for best results. It's finding a balance between all four jetting circuits is what you are looking for with good transition from one stage to the next. I believe the proper order to jet is 1st-PMS, 2nd-Pilot jet, 3rd-Main Jet, 4th-Needle height. Repeat and retest as necessary. Many recommend setting Needle 3rd or before setting the main jet. I must disagree, because all fuel that is controlled by the needle must first travel through the main jet, When ever the main jet is changed, it can cause quite a bit of an affect upon the needle jetting and you'll end up chasing your jet sizes in circles causing yourself lots more grief, frustration, confusion and time.

Alternate main jet sizing method (Speed Jetting)
Record either elapsed time or top speed over a set distance. Keep increasing main jet size until a drop in speed or increase in ET is noted over that set distance, then drop back one jet size. Weather should be a normal clear day, average temperatures. This only works when sizing the main jet with wide open throttle while the engine is under load.

For instance, say your top speed was 45 mph for a distance of 200 yards. Change jet and run again. If you start out a where you are sure you may be a little lean, speed will peak out at optimal jetting and then start to slow as you get too rich again. Distance should be long enough to easily determine a difference, but short enough to not top out engine RPM and belt upshift (distance short enough where its still pulling hard at the finish line) Your top speed and/or elapsed time over a set distance will vary with main jet size. The more stable the temperature,wind and the weather when doing speed jetting like this on the main jet the better. As barometric pressure changes, wind and temperature changes, it will affect your results. If you have a GPS, some of those will record and hold maximum speed. Quite a handy function. KEEP NOTES
Example runs.
45 mph with 145 main
47 mph with 147.5 main
49 mph with 150 main (maybe best jet to use, not too rich or lean.)
48 mph with 152.5 main (Good jet for year round use, with COLD winter temps)
47 mph with 155 main

I got to where i could pull the belt exhaust duct work off leave it off while jetting and just rotate the carb in the boot to change main jet from the right side on the trail, about 5 minutes max to change it.

Once your jetting is set for optimum power, you can feel it, it is a rush. And nearly free HP. Individual jets are cheap to purchase. However, Optimum jetting will be right only for a narrow range of altitudes and a narrow range of temperatures. If you ride in wide variations of temperature and or altitudes without re-jetting, you may do damage. Especially if you jet for optimum power when it is a hot summer day then ride in the sub zero winter temps. Yes jetting is time consuming and a pain in the ass to do it right. If you want to maintain maximum performance year around, you may need to re-jet. With experience and good documentation, it is easy to seasonally re-jet once you develop a history. If you compete, (mud bogs) optimized jetting for the events altitude and expected temperature can give you an performance edge many do not bother with.

If you don't want to bother with seasonal jetting. Jet a little rich for about 1/2 of your expected seasonal temperature extremes and you may be close enough year around with just tweaks on the PMS screw. For example if your seasonal temperature ranges from 0 to 90 degrees, mid way is 45 degrees. The only time your jetting will be spot on and getting maximum power is at this temperature. As it warms up you will be richer. As it cools down you will be leaner. Plug chops should be performed to insure A/F ratios are safe. There is also EFI for those who hate jetting too.

V-Twin jetting
Procedure is identical to a single cylinder engine. You have two carburetors to jet instead of one. Not sure if its the same on the AC 650 liquid cooled twin or not, but on every air cooled V-twin motorcycle I've jetted the rear cylinder usually ended up being jetted 1 main size larger, needle raised about 10 thousands higher and the PMS screw open about 1/4 turn further then the carburetor for the front cylinder. The reason for this is the rear cylinder gets less cooling then the front and the rear cylinder usually breathes better since the distance from the exhaust outlet and the tail pipe exit is shorter.

Carburetor Balance on V-twins
Synchronizing the carburetors (ensuring each is sucking the same amount of air) at idle or just above idle really smooths out the engine and increases power. At speed this results in less vibration and better cylinder power balance. Really noticeable on a V-twin motorcycle. You can feel it in the handlebars or on the foot pegs when it's out of balance. You want both cylinders to share an equal amount of load placed upon them. This is done by adjusting a screw between the carburetors while watching the intake vacuum of each cylinder. Adjust the balance screw one-way or the other until both vacuums are equal. Many times a 1/8 turn or less makes quite a bit of difference. A U-tube oil filled manometer is perfect to use for this where it shows the overall difference in intake vacuum between the cylinders. Cheap and easy to build out of plastic tubing stapled or taped to a support. Here is a link to one made from tubing and a yardstick. A scale is not important and it doesn't need to be that long either. Mine is about 16" long with 4 ft of 1/4 inch ID tubing stapled into groove routed 3/8" deep in a 1x3 pine board. I also used a couple old main jets of about 110 in size to minimize pulsing of the oil column inserted into each end of the tubing. Slipped over the top of each end is about 4 ft of 5/16" id plastic tubing that runs to each vacuum port on the intake manifolds. I just hang the board from the handlebars.

PMS screw settings have an impact on Carburetor sync balance and vice versa. When setting Carburetor balance, start with good PMS settings, then after setting balance, recheck PMS settings. Vacuum reading differences between an idle RPM and higher RPM's around 2000-2500 should remain close to equal and balanced once you have both the PMS screws and the carburetors synchronized to each other for both cylinders. If differential intake vacuum changes when at higher RPM's then it is at idle, you have a miss match between PMS settings and Carburetor balance setting. Be persistent. You'll get it.

General Jetting Comments
Setting jetting the same as your buddy, or what's recommended by a vendor will probably get you somewhat close, but doubt of you are going to get maximum power for your setup. (Intake & exhaust modifications, engine efficiency & carburetor tolerances, ring seal, temperature, altitude, humidity are all variables) For setting the PMS screw, it's best to use an extended reach right angle carburetor adjustment tool. Motion Pro makes one. Google it. If not you better be wearing a leather glove, have a small hand with a very short screwdriver and be quick or you'll get burned.

Adding choke and getting improved performance is a guarantee that you need to go larger in jetting somewhere. You can test both the needle and the main jet this way, Main jet size via acceleration with wide open throttle and needle height via acceleration performance or time from standstill to 5/8 open throttle. (Set the throttle limiter screw on the handle bar down so you do not exceed about 5/8 open throttle) The inverse is also true, if the acceleration is better to 5/8 open throttle with the air box lid off then the needle is rich, or if the acceleration is better with a wide open throttle with the air box lid off then the main is too rich. Remember fuel delivery is determined by the amount of throttle opening while the engine is under load, not engine RPM or speed. You are balancing the amount of fuel delivered into the engine at various amount of air intake (throttle opening) into the engine. If all your jetting performance testing is done only at wide open throttle, you are just testing the main jet. You need to check performance from closed throttle to 1/2-3/4 open to size the needle height correctly. AGAIN plug chops should be performed while the engine is under load only for best results. The engine must be under load but not lugging. A CV carburetor will self lean out when the engine is under light loads. This is normal and desirable for best mileage but it does make plug chop reading a little more difficult when jetting jets for best performance. To place the engine into load, find a hill with a pretty good incline to it. Performing a Plug chop and reading the plug when the engine is lightly loaded (I.E. cruising 1/2 throttle on level ground) will not give you proper results when performing plug chops to size the needle height. FIND a HILL.

On v-twins, Those very skilled in plug reading, or those who have measurement equipment to read actual A/F ratio, or those who have the ability to measure cylinder temperature are usually the group that jets the rear cylinder main 1 step higher. And maybe the rear needle 1/2 washer thickness higher as well. Its rare to see the rear cylinder of a Vtwin engine not jetted a little larger then the front even from the factory. Jetting in the motorcycle community on stock engines is usually a lot closer from the factory whereas in the ATV world, close is usually good enough. I'd say the odds are in favor of reaching better performance and balanced load upon the cylinders with the rear cylinder a little richer though, but as in all things with carburetors not always. Carburetor synchronization, Airbox air flow and also manufacturing tolerances of the carburetor it's self, throttle cable length (front to back) and jets used all play a factor here. Throttle cable lengths and synchronization screw adjustment can be a huge factor here. Is both carburetors 1/2 open when the throttle on the handle bar is 1/2 open? if not then each cylinder isn't sharing the load equally.

Once you get jetting to where you want it first insure your carburetor is synchronized and then get a hold of a measurement device to read cylinder temperatures, if the rear is running hotter by more than about 15-20 or more degrees after sustained full throttle riding, or so 1 size larger main jet will probably help equal out the temperatures and help balance the load placed upon the engine cylinders more equally. If there are performance increases it may be small and barely detectable with your Levi dynomometer but it will be easier on your engine and give the plug life of the rear cylinder longer life and a little better ET. You can also check the rear needle height with temperature measurement with sustained riding at 1/2 throttle and comparing cylinder temperatures.

(Note: Many or Most vendors that recommend jetting sizes, will always recommend jetting that is too rich for optimum power. They will always error towards too rich then too lean. A good vendor does not want to be responsible for a burned up engine. The same goes for factory jetting it is usually to rich also) Jetting from the factory is for sea level. Most dealers do not re-jet if you live at higher elevations. Any change in intake, exhaust or volumetric efficiency of an engine may require re-jetting. Dyna-jet kits that supply replacement needles are a good investment. Dyna-jet claims their needle tapers are optimized on a dynamometer for the intended engine application. I tend to believe them since 90-95 percent of the time a Dyna-jet needle alone results in better performance then stock needles. Keihin carburetors have a 1-size fits all approach on the needle regardless of what engine the carburetor is installed on.

When doing plug chops.
Performing correct plug chops is critical in getting good color results on your plug. Use new or freshly cleaned plugs only. Dirty plugs will not work. You'll need a clean plug for each plug chop for each carburetor circuit for every test. (A cleaned plug should look like a new plug) Hold the throttle at the condition you want to inspect for several minutes, (1/4, 1/2 or Wide open) On an Automatic, slightly reduce throttle to remove engine load, shift into neutral and immediately hit the kill switch. Continue to hold the throttle at this test condition and coast or brake to a stop. If you back off the throttle, engine brake or idle the engine as you come to a stop, you are not locking in actual test throttle position to read your plug and most likely just reading the idle condition over and over again. On a manual transmission, if you press down on the shifter and hold it, the clutch will disengage and you can coast or brake to a stop. Don't try to shift into neutral, just hold the shift lever down. Use a gear that gives you good load on the engine, where you are not bogging it or hitting the rev limiter. An engine must be under load to read a plug. Best results are obtained on a slight uphill slope. Some have success in performing plug chops in low range to reduce speed, but use a steeper hill to fully load the engine. Never attempt plug chops by just revving the unloaded engine for extended periods in neutral. Doing so can destroy your engine and gives poor color results. Plugs chops can be dangerous for both you and your machine. Be very careful. Practice slow speed plug chops before you attempt higher speed plug chops.

Reading the plug for mixture.
Plug reading was a lot easier when we all burned un-oxygenated leaded gasoline. The lead in the gasoline would color the entire exposed ceramic a nice golden brown when jetted correctly. You can still read plugs with today's fuel, but do not expect to see the same color results as you did in the good old days. When doing plug chops use a new or cleaned plug. The plug ceramic, the exposed ceramic should be almost clean or very light lighter near the tip and getting darker as you look deep inside the plug. To error on the safe side, a light brown deep inside is probably the color you want there and the part that is easy to see will almost remain white. There will be a thin dry black deposit on the base ring. If you're getting heavy fluffy black deposits on the base ring you're probably too rich on the fuel metering circuit you are testing. You never want to see shiny specks of silver on any part of the ceramic, if you do you are too lean. Those silver specks are from melted aluminum off the top of your piston. Gasoline additives and brands can affect plug color. Those using high-octane leaded race gas, performance cams, or high compression pistons may get darker colors when jetted right. The plug reading tips here are for un-leaded fuels only on lightly modified engines. Nitrous oxide will give lighter plug color and during the NOX boost, require a lot more fuel to prevent the engine from going dangerously lean and melting the top out of the piston.

For reading plugs, the link below is a pretty good guide of what to expect when doing plug chops with new or cleaned spark plugs. On Arctic cat engines the best power is achieved when the plugs look like the ones in this link labeled as "Slightly Rich" or on the safe side a tad closer to what they label as "Too rich" Although the link below refers to Drag strip automotive use, it's still a pretty good guide for any 4-cycle engine burning today's fuel. Remember it takes time to build color on the ceramic. If you're getting lots of color on the ceramic on new plugs right away, chances are you're too rich.

Understanding Carburetor Jetting Circuits.
When at idle, Most of the fuel is supplied from the PMS screw amount of turns open.

When at slow speeds or less than 1/4 open throttle, most of the fuel is supplied from the Pilot jet in addition to the amount supplied by the PMS screw. The needle is just beginning to add fuel to the jetting at this point.

When at throttle openings greater then 1/4 open to about 3/4-7/8 open, most of the fuel is metered by the needle taper and the amount of distance the needle is raised out of its opening, In addition to the amount of fuel supplied by the PMS screw and the Pilot Jet.
There is an actual needle jet that is of a set diameter that held into place by the main jet retainer (holder). There has been a number of people who have found this needle jet missing. They either lost when the carburetor was diss-assembled or it was lost by previous owner. If you remove the main jet retainer for carburetor cleaning, make sure you note the proper installation of this jet, note distance it sticks up through the throat of the carburetor first. You can install it upside down. Removing the main jet retainer is an important step in a good carburetor cleaning. There are tiny air bleed holes that atomize the fuel into a mist. If the air bleeds are clogged it can affect jetting and power delivery. There are also tiny air bleed holes on the pilot (slow) jet.

When at WOT most of the fuel is metered by the main jet size only, with influence caused by needle height setting. In addition to the amount of fuel supplied by the PMS screw and the Pilot Jet.

All 4 circuits (PMS, Pilot, Needle & Main) overlap; adjusting any one of the fuel circuits will have influence upon the others. Each successive circuit will add fuel to the amount supplied by the previous fuel metering circuit. Get everything set right and you'll get maximum power at all throttle openings for your altitude plus or minus a thousand feet or so. Document your progress carefully. It's best to make one change at a time.

See the jetting fuel Percent contributions drawing below. What this picture attempts to show is the percentage contribution of total fuel required delivered by each of the 4 jetting circuits at different throttle openings. The scale and height of each color is not accurate. I've included it just to show how the 4 jetting circuits overlap and how the PMS setting, Pilot size & needle setting can have a small affect upon total fuel delivery requirements even at WOT. Look at the relative height of each color band as a Snapshot percentage of contribution for various throttle openings instead of a linear transition from one stage to the next. For instance, At WOT 85% of all fuel required comes from main jet sizing, but still 5% controlled needle height, 5% controlled by PMS setting and 5% controlled by Pilot jet size. At 1/2 open maybe 10% PMS, 10% Pilot size, 60% needle height, and 20% main size. At 3/4 open 5% PMS, 5% Pilot 30% needle, 60% main. This is why I recommend retesting all jetting circuits via plug chops when ever any jet is changed. Percentage of total fuel requirements delivered by each jetting circuit overlap and have influence upon each other.

However when troubleshooting if you are experiencing problems at Idle RPM's then you would adjust or change pilot jet. Suspect plugged pilot jetting or the tiny posts and passageways in the carburetor body and those tiny air holes in the bottom half of the main air inlet throat or bore, Choke (enrichment) plunger leaking, Idle speed adjustment?

If you are experiencing problems at mid throttle, then you would raise or lower the slide needle position. Or suspect problems with the Diaphragm, Air bleed holes on the side of the main jet retainer plugged? Carburetor vent tube plugged? Assembled incorrectly? Missing parts? Slide sticking? Choke (enrichment) plunger leaking

If you are having problems at wide open throttle. That is the main jet. Or suspect problems with main jet sizing, plugged? Main jet swapped with Start jet? Air bleed holes on the side of the main jet retainer plugged? Water in float bowl? Choke (enrichment) plunger leaking

All jetting circuits, Pilot, Needle and Main are greatly affected by float bowl fuel level and fuel delivery. And air delivery. Is air filter clean? Are air passageways to the air filter adequate and clean?

If the fuel level in the float bowl is set wrong, its more difficult to get any of the jetting set right.

If fuel delivery isn't correct the engine may starve for fuel as the engine may be consuming fuel faster then the the fuel delivery can keep it full? Suspect clogged fuel filter(s), Vacuum lock? Pinched fuel lines, Poor fuel pump or other restrictions in getting fuel from the tank into the carburetor.

In every Keihin Carburetor I've seen the PMS or idle mixture adjustment screw adjusts the amount of fuel, Clock wise less fuel and more lean, CCW more fuel and richer as viewed from the screw head. On every Mikuni carburetor I've seen the PMS or idle mixture adjustment screw adjust the amount of air entering the engine at idle RPM's CW less air thus richer. CWW More air thus leaner.

Common Mistakes.
Not getting the diaphragm sealed correctly after changing the needle. Make sure slide travel up and down in the carburetor is complete and not sticking, just lift it with a fingertip reaching into the throat of the carburetor. Assembly of the needle & needle retainer wrong. Check the parts diagrams. Mixing up the Main and the Start Jet, they both look the same, and will interchange. The Start jet is the littlest one in number. Make sure it is in the right place. The main jet is always directly below the needle. Make sure jets are tight, but don't over tighten and be careful not to cross thread. On the PMS screw, a VERY light coat of grease on the threads will keep it from seizing into place. Since the PMS screw is recessed, pack the recess with thick grease when you done. This helps keep grit out of the threads.

Pinching off or twisting the fuel supply line can be a big headache when it goes unnoticed. A carburetor that is loose in the intake boot can also be a major pain in the butt causing a very lean running condition. On many engines the fuel pump is vacuum driven mechanical diaphragm pump. Its' quite easy to damage or kink the vacuum line from the intake manifold to the pump. Make sure the throttle is working freely after you install the carburetor back into the intake. You sure don't want to start that engine with the throttle stuck in a wide-open position

Tip on cleaning plugs
Never clean a plug with a metal wire brush. Metal from the brush will be transferred from the brush wires to the plugs ceramic possibly shorting out the plug. Think about it, many sharpening stones made for hunting knives are made from ceramic. Sand blasting is the best way to clean a plug. Small plug cleaning sand blasters can be found on Ebay for around 10-20 dollars, a good investment for your motor toys. Carburetor cleaner and old toothbrush works pretty good for cleaning a new plug between plug chops.

To determine proper heat range.
For heat range, the color change on the ground strap should be midway on the length of the ground electrode as it shows on the picture below. Until heat range is correct, setting mixture can be hit or miss. Engine modifications, especially cams and porting, intake and exhaust changes where more fuel is dumped into an engine may end up requiring a plug with a different heat range. Usually hotter since the increased volumetric efficiency of the engine cools the plug down easier but not always. A plug that is running too cold will tend to show as a rich mixture on color, even when the mixture is leaner than it should be for best performance. Vice versa on a plug that is running too hot. 90% of the time, the stock recommended plug is correct, but worthy of ensuring it is correct just to eliminate jetting errors.

If a too cold a heat range is used, the color change on the ground strap will be closer to the end (the tip over the electrode) of the ground electrode and plugs tend to foul out quicker. When checking plug color, it will appear to be richer then it really is.

If too hot a heat range is used, the color change on ground strap moves towards the base of the plug. (Closer to the weld) The plugs center wire electrode tends to burn off increasing gap distance faster and wearing it out. Also a too hot plug causes pre-ignition of the A/F mixture, which can cause mechanical damage in the engine over time. Detonation puts a lot of stress on an engine and may cause backfiring through the carburetor. When checking plug color, it will appear to be leaner then it really is.

Ignition advance
Is shown by a color change on the wire electrode for a length of about 1/32". Since timing is not adjustable on cats, no need to go into it with any detail. Offset magneto flywheel keys or after market ECU's may change ignition timing and timing advance curves. So if you are using one of those devices, it might be a good idea to research into it more. Google it if you wish, for more information.

Tip Plug Indexing
Indexing the plug, or insuring the end of the ground electrode (open end) points towards in intake valve has been proven on a dynamometer for many engines to increase HP by up to 1 to 1-1/2%. Indexing is done with shims under the plug gasket. Mark the outside of the plug to show the orientation of the ground electrode and shim the plug to insure it points towards the intake valve. This promotes faster flame propagation and improved overall mixture burning. Indexing shims to fit plugs can be found on many motorcycle speed shop websites.

An almost worthless plug tip.
Higher altitudes require less plug gap. Since the air is less dense and cylinder pressures are lower. The mixture has a higher resistance. Thus making it harder for the spark to jump the gap. Closing the gap down about 1% per 1000 ft increase in elevation will many times improve a plugs performance and life at higher altitudes. Given a plug normally gaped at 0.028 at sea level, it would be gaped at .025 to .026 at 8000 ft for best performance. Not much of a change and nothing to worry about if you're not up that high in altitude. I first learned this trick on my 73 Chevy 350 pickup that recommended plug gap setting of 0.060 and lived at 7000 ft. Setting plug gap on that engine to 0.055 reduced engine miss, increased MPG and improved performance.

Higher altitudes allow further ignition advance
If you have an after market ECU that allows you to adjust ignition timing. You could probably increase timing further then sea level recommendations. For instance if sea level timing was 10 degrees advancing to 30 degrees with rpm, for high altitudes a timing curve offset may help you develop power say 13 degrees advancing to 33 degrees. This is not intended to be a hard fast recommendation, just an old tuners trick that was common in the past on adjustable timing ignition systems for about any engine. Works best when bumping up a grade in gasoline octane as well. Result in a little slower burn over a longer crank rotation thus getting more power from the power stroke cycle. A few vendors sell offset keys that can be installed on the crank to rotate the flywheel to offer more advance. Many report improved performance as a result.

A useful Plug tip that gives merit to the worthless plug tip.
Sometimes a plug will just not fire or spark under compression. Removing the plug and watching the spark jump the gap looks great in the open air, but just will not run at all when installed. The plug spark is shorting out internally instead of jumping the gap. This can be a real bummer when out on the trail miles from home with no spare plug. Try closing the gap down to where its gaped closer this sometimes works and will get you home. I highly recommend carrying a spare plug set in a case where they will be kept clean and undamaged instead.

Performance Plugs
In my opinion precious metal plugs are sometimes worth the price, either platinum or iridium. Why, first they rarely wear out in a typical life span of an ATV engine, and in my experience they result in easier starting and a smoother idle too. Dynamometer results do show a slight increase in HP with most performance plugs on most installations. May be less than 1% though. CR7EIX is a NGK Iridium replacement that works pretty well in most Arctic cat engines. The Denso IU22 is another (direct cross over from the NGK CR7EIX. You can also get Denso plugs Hotter or colder then the IU22 such as IU20 or IU24 if you need to go up or down a heat range. Stock plug for most Arctic cats is either the NGK CPR8E or the CR6E either will work, the CPR8 E is actually two heat ranges colder then the CR6E and still is the correct replacement because of the Extended Nose construction which naturally runs hotter.

Hotwires & Performance coils
Performance add on's available for ATV's. I've no experience with them; I've read as many negative reports as I seen positive reports. I have yet to try them myself so I have no opinion. Some say its snake oil and clever marketing to take your money and some say it's the next best thing to sliced bread? I don't know. I would have to see dynamometer runs to be convinced that they help. Funny the people who build these gadgets never show actual dynamometer results to back up their claims. So maybe there really isn't any gain. I've read a number of posts on automotive and motorcycle tuners forums who claim Hotwires result in less HP on a dynamometer especially at higher RPM's. Although many (even those who say they lost HP on a dyno) also claim easier starting and a smoother idle with hotwires. If they (the Gimmick makers) want my money, SHOW ME real DYNO DATA. Prove it is better. Most will not show actual dyno data but go to great lengths showing everything else, videos, dark room spark testing and fabricate up false claims instead of actual Dyno data. What are they Hiding?????

A performance coil may be a good replacement option if you ever need to replace a bad stock coil. Many times they are cheaper then a new stock coil from the dealer. I don't think I've ever read where a performance coil gained noticeable hp gains (feel) over a good stock coil. One may see actual gains in HP on a dynamometer however using a performance coil. It's like plug indexing and precious metal plugs it all adds up if you're looking to maximize performance.

Drilling the Slide
Many who have done this on non Dyna-jet equipped carburetors report improved performance and better throttle performance. If you are using a Dyna-jet kit, chances are the Dyna-jet instructions already recommended that you do this. Some Dyna-jet kits include a drill bit, however not all Dyna-jet kits do. There are some who have done this on Dyna-jet kitted carburetors when Dyna-jet did not supply the drill bit for drilling the slide also had good success. Keep in mind, if you deviate from what Dyna-jet recommends, then their jetting recommendations will also probably be invalid. If you pull the slide out and look at the bottom, you'll see 2 holes. One dead center, that is where the needle is installed, the other is offset from dead center. This is the hole you enlarge to 1/8" Do not drill the center hole. Drilling the slide allows the slide to open quicker since intake vacuum has a path of lower resistance to top of the diaphragm to lift the slide. Raising or lowering the needle with shim washers after drilling the slide may be required to achieve best results.

Plugs chops in the middle throttle positions should be performed to insure the mid range is not too lean and adjust needle height accordingly. The engine must be under load but not lugging. A CV carburetor will self lean out when the engine is under light loads. This is normal and desirable for best mileage but it does make plug reading a little more difficult when jetting jets for best performance.

Clipping the spring
This is a modification for stock slide springs only. This mod or idea originated from copying the replacement springs that are contained in some Dyna-jet kits. Some of these kits include a shorter spring then a typical stock spring. The length of the Dyna-jet replacement spring is approximately 4 coils turns shorter then a stock spring. If you are using a Dyna-jet kit, and the kit did include a shorter spring, I would not cut a short Dyna-jet spring shorter.

So take it as it currently stands. The two modifications together seem to work for the most part for most people on a stock carburetor. If your carburetors mid range is already close to giving good A/F ratios in the middle throttle positions for your altitude & atmospheric conditions, the modifications may not deliver as much performance increase as it does for others. Dyna-jet kits are good. They seem to have their act together and put together quality jetting kits. The needle alone sometimes greatly improves performance. Do I recommend you to drill and clip? Maybe, and less so recommended with a Dyna-jet kit installed already. If you do, proceed under your own risk and make sure you do plug chops to make sure you don't end up too rich or too lean at mid range throttle openings. Worse case, you'll have to purchase a replacement spring and or slide to get back to stock conditions.

How to do the slide Mod
Remove or rotate your carburetor sideways to obtain access to the 4 screws holding the diaphragm cover in place.

(Note on my 06 H1 I can get access to the carb slide just by loosening the clamps on the intake and outlet boots and carefully rotating the carb 90 degrees sideways towards the right. Be careful with throttle cable, electric choke wire and the vacuum impulse line to the fuel pump if you do it this way. Belt air air outlet will be taken loose at the bottom and pushed up and forward out of the way. Yours may be different. )

Remove diaphragm cover. Underneath is a rubber diaphragm, spring & slide. (Careful, don't' loose parts since spring loaded.)

Remove diaphragm and slide. Carefully noting how it all comes apart. The needle is held into place by the spring and spring retainer only, there may be washers and other tiny parts involved. Note the order everything comes apart.

Cut 4 coils from the length of the spring. Discard the short piece.

On the slide look at the bottom there are two holes a little less then 1/8" diameter, One dead center (Where the needle pokes through) the other offset forwards about 1/4-3/8 inch maybe.

Drill the offset hole larger with a 1/8 drill bit clean any burrs and drill shavings. DO NOT DRILL THE CENTER HOLE

Re-assemble everything in the exact order it came apart. Getting the diaphragm stuffed back under the cap may be a real pain in the ass, if it leaks it will not run right. Be persistent. Sometimes it helps to put the diaphragm in the freezer for a while this stiffens up the rubber making it easier to control.

Note: Depending upon how rich your carburetor was on the needle before this mod you may be able to get better performance by raising your needle height. This will enrich your mid range. If so use #4 stainless steel machine screw washers to shim the needle so it rides higher in the slide. Each washer is about 20 thousands of an inch thick. One or two may do it, believe you could use maybe up to 4 washers if you had too. Plug chops or O2 testing equipment with the throttle 1/2 open will determine if your needle is rich or lean, Jet a tad rich on the needle for best results. No hard fast recommendations on the number of washers to use, you just have to try it at different heights to see what works best.

(Chances are; [depending upon your current jetting and altitude] one or two washers raising the needle will be better than stock. On Dynajet adjustable needles, the 3rd, 4th or or even 5th notch from the top seems to be the sweet spot for many. )

The screws used on the slide cap are cheap and screw slots will strip out easily, good time to replace with 4 stainless steel allen head screws from hardware store. Just match thread and length, I've replaced the 4 on the slide cap and the 4 on the carb bowl myself.

Not running right?
Many times an ATV not running right, can be caused by dirt, grit, water, gelled old gasoline inside the carburetor if it is not a re-jetting error. A good complete dis-assembly and cleaning will be required. Nearly all the problems shown below can be traced back to a dirty carburetor also. If you do a half-assed job cleaning a carburetor, expect half-assed results. The following link shows the tear down and reassembly step by step. Your carburetor may be a little different then the one shown, but close enough and the steps will still apply. Now that you've gone to the trouble cleaning a carburetor, lessen your chance of having to do it again by installing a in-line fuel filter in the gas supply line to the carburetor. Once again unless you completely tear down a carburetor to clean it, you are only doing a half-assed job of cleaning.

Does it run better at a particular throttle opening with the choke on (Enrichener valve open)? If so, that circuit is probably lean. No manual choke, close off part of the intake air tubing to allow less air into the engine. Does it run better with the air box lid off? If so that circuit is rich. What does your plug tell you?

Back firing on deceleration A LOUD explosive backfire is what you don't want. Chances are you miss read the plug for 1/4 throttle opening and the pilot jet and the PMS is too lean. Did you perform your Plug chop under load? Some popping and rumble is normal and to be expected. With exhaust modifications or aftermarket exhaust, it's quite loud and yet normal for a good deceleration rumble to take place when jetted correctly. If you don't like this rumble, you can make it less noticeable by setting the pilot and or the PMS richer. You may loose some performance off the line but it wont hurt anything as long as you don't go too rich and start fouling plugs. Quite often raising the needle higher to enrich the mid range is also needed to get rid of the explosive backfires out the exhaust pipe.

Jetting Chart
The altitude-temperature correction chart is easy to use. It is based upon a standard of 68 degrees at sea level. Therefore, at these standards, the correction factor is 1.00. This is the factor you read on the left scale where the green line intersects the 68 degree vertical black line.

Now if you are at sea level now and currently jetted spot on when the temperature is around 68-70 degrees and you're going to ride in the Rocky Mountains at 9800 ft at 68 degrees Follow the vertical black line to the Purple line and read the correction factor on the left scale. In this case, about 0.92 (read between the lines if you want and estimate) This means to be jetted correctly for the high altitude you would need to decrease jetting by applying the new factor to current jet size. For instance, if you presently have a 150 main jet. Multiply 150 X 0.92 and you get a 138 main jet to ride at 9800 ft and at 68 degrees or about 8% leaner. Make sense?

Jetting for Cold weather can make a difference too, especially if you wish to maintain maximum performance as it gets cold. Keep in mind the temperature of the air entering into your engine is whats important. Not the temperature outside. The factory air-box on ATV's will warm the air quite a bit from engine heat.

Jetting for cold weather is pretty straight forward. Using the jetting table below, If you were jetted perfect for your altitude and at 85 degrees F then at -5 degrees F then jetting needs to be about 10% richer for optimum performance. What is 10% richer? The area size of the hole in the jet needs to be 10% larger. On Keihin jets just take the jet size and multiply by 1.1 to obtain the new jet size. Keihin's numbering is equal to the diameter in centimeters. A 140 jet has a diameter of 0.140 centimeters.

For instance, if you were jetted spot on at 70F at sea level with a 140 main jet and to be running close to spot on at -5F, Take 140 * 1.1 = 154. On the jetting table, your base condition is where the green line intersects the 1.00 and the 68F vertical line. At -5 degrees, follow the green line up to the left until it intersects at -4 and 1.08. This is a difference of 8 percent larger jetting needed for the 80 degree change. If you were jetted perfectly for 85 degrees the green line is at the intersection of 0.98 and 86F Jetting for -5 degrees shows the green line to intersect at 1.08 and -4. the difference in correction factor is 1.08 and .98 or 10% (See (Temperature jetting on an ATV))

To jet for Altitude change it is a little different. Lets say you were jetted perfectly at 85 degrees at sea level and you want to re-jet for 6500ft. In this case, using the jetting table below your base condition is found on the green line where it intersects the 0.98 and the 86F. For 6500ft foot at 85F the blue line is used. Where it intersects at about 0.9225 and 85F or about 5 3/4 percent leaner for an altitude change only.

If also jetting for a colder temperature at this 6500 ft altitude say 32 degrees, follow the blue line up and to the left to where the line intersects at about 0.99 and 32F. Now compare the original correction factor of 0.98 to the new correction factor of 0.99 and this shows the new requirement to be only about 1 percent richer required then your base jetting of sea level and 85 degrees. It can be a little confusing at first until you get the hang of it.

Since the number scheme of dyna-jet jets are different you first need to know the diameter of the hole in the jet and then Multiply the diameter first to determine the new required diameter and then look up the correct jet to use for the new diameter size.

Many including myself jet a little richer than optimum for warm weather so when it does get very cold its still plenty rich to run well enough. I'm jetted for an optimum temperature of about 40-45 degrees and that is where I'd get best performance. But when it's -5F its still good enough and when its 85F also still good enough. Going by the jetting table I would be about 5% too lean at -5 degrees and about 5% too rich at 85 degrees. Only adjustment I'll make seasonally is to open the PMS screw about 1/4-1/2 turn more when very cold and close it back down to where it was when it warms up. This gives me a smooth idle and won't die while warming up.

(Temperature jetting on a CV carburetor or ATV)
I've included a jetting table that was made for snowmachine carburetors and mainly for 2 cycle engines which are a lot more picky about proper jetting then your average 4 cycle engine is. For equal displacement a two cycle engine burns about twice as much air as a 4 cycle does. I've used this table with success on my 650H1 although what the table recommends may end up being a little richer than ideal. Knowing this I'll usually drop back about 1/2 from what is recommended for temperature change. In the above example where the table calls for a 154 jet I'd probably use a 147.5 instead. I've got a very short 2" ID air intake on my 650H1. My Intake consists of 1-90 degree ell and 1-45 degree ell and about 4 inches of PVC pipe. Air intake is on the side of the Cowling where it Says "Arctic Cat" So I am getting colder air into my engine then I would with the stock factory air ducts or a snorkel that has it's intake air piping running close to the hot engine. So is the jetting table any good for your ATV? maybe, maybe not, you'll have to try it and see. You'll probably find if you adjust the recommendation to about 1/2 of what is recommend for temperature changes only it may be closer to ideal. I have found that the recommendations for altitude to be very close though.

It should be obvious now that each horizontal division is a change of 2% in jet size where your jet requirements are leaner for higher temps and higher altitudes and richer for lower temps and lower altitudes. As the weather changes and you notice your engine is running better or worse, now you know why and this chart gives you a starting point to make jetting changes. Once optimum jetting is found for any single altitude and temperature, this chart will get you very close to optimum jetting for a change of temperature altitude or both on almost any engine. Back it up with your own documentation.

A tip that can keep you jetted closer longer as altitude and temperature changes.
During colder weather, open the PMS screw up to 1/4 turn maybe as far as 1/2 turns further open, this will help enrich all the fuel circuits, just close it back down when it warms up in the spring. Also when at higher altitudes you can close the PMS screw up to 1/4 -1/2 turns, this will help lean out all fuel circuits when at high altitudes and less fuel is required. Remember to open back up when your get down to lower altitudes or you're going to be too lean. Adjusting the PMS screw open or closed from your current setting will just about make the difference of 1 full main jet size or about 1/2 size jet down and 1/2 size jet up from your present ideal setting. It's often not quite enough but it helps and gets you closer and keeps you from re-jetting all the time.

More Comments
This is only a guide, not a hard fast recommendation, plug reading for jetting is something that takes experience and practice. It is an art. The pros use a magnifying glass and strong light and study the plug up close from all angles and deep inside. 90% of those who claim to be able to read a plug cannot. Sad but true. I've been reading plugs on motor toys for 45 years and I'm no plug reading expert. 95% of the pictures on the Internet and guides for proper plug reading are complete garbage. Further proof that many do not know how to read plugs. No information would be better then BAD information in this case. Most Internet plug pictures I've seen are pictures of plugs that were well used & worn out. Probably pictures of plugs used in an oil burning 2-stroke engine and when we all burned leaded gasoline. A 4-cycle engine burning un-leaded fuel with MeOH added will color a plug much lighter and with less color when jetted correctly. It was the lead in gasoline that developed that nice even golden brown color on the porcelain that many pictures show as correct jetting for plug reading. If you are trying to get a similar golden brown on all of the exposed ceramic you'll probably never see it unless you are very rich and/or fuel with odd additional additives. Or you're using leaded high-octane race gas.

Plug reading after a plug has several hundred miles on it gets a lot easier and looks like the picture at the bottom left of this post when all is well and good. Problem is you can do a lot of engine damage if you wait hundreds of miles for color to develop on a plug if you are too lean.

If you ever want to see what good nearly perfect plug color looks like under what you could call perfect combustion, next time you put new plugs into your stock computer controlled closed loop fuel injected automobile engine. Drive about 10-20 miles on the highway at a steady speed, do a plug chop as described above, pull over and inspect a plug. You'll see the results locked in for that throttle position. It will show no or almost no color and very little if any deposits. That is if all emission circuits are working correctly. But bare in mind your closed loop EFI automotive engine electronics try to maintain a perfect stochastic ratio of 14.7 to 1 Air fuel ratio. Although 14.7:1 is great for emissions and fuel economy it doesn't deliver the best power. Best engine power is developed somewhere between 12.5:1 to 13.5:1 A/F ratios depending upon engine design. So optimum jetting for power should show some color and some deposits on a new plug. I have an Innovate wide band O2 sensor that I use as a jetting aid. My 650 H1 will run at mixtures as rich as 10:1 and as lean as 16.5:1 and run pretty good too. Beyond these extremes it tends to miss and run rough. In my opinion, prolong operation under load and lean above 14.7:1 may cause damage long term and damage would happen sooner the leaner it is. Maybe within hours at 16:1?? I've also found my 650 H1 likes an A/F ratio of about 12.8-13 to 1 for best power based upon my calibrated "Levi Dynamometer". For my altitude and modifications, a 150 main, 20 thousands shim under the stock needle, 45 pilot jet and 1 3/4 turns out on the PMS is pretty damn good base for around 45-50 degree weather. For temperatures in the 80's I'd probably get better performance with a 145 main jet. I purposely jet rich so I do not have to seasonally rejet as often.

Using the 42.5 pilot, I will get an occasional loud backfire when closing the throttle at speed, I can greatly reduce the tendency to backfire by installing the 45 pilot. But according to my wide band 02 sensor a 45 is too rich for best power off the line. I've installed a 158 main jet and open the PMS to 2 1/2 turns in the winter to blade snow when it gets way down below zero. Sometimes -40 even. Yes temperature makes that much difference. This year, I've also experimented with closing off approximately 1/2 of my air intake (air jetting for winter time use) so far this appears to be working pretty well because it doesn't require winter jetting changes. My cold air intake is a 2" PVC elbow that exits outside the front plastic right above the shift lever. For air jetting I am trying a 1 1/2" and 1 1/4" slip in PVC bushings. Much easier then rejetting the carburetor. I think its' going to work well, but I do suffer a performance loss doing this that I wouldn't have if i rejetted instead.

Notes on EFI

14.7 to 1 ratio is theoretically when all air and all fuel is 100% consumed in the combustion process. A ratio Higher than this is lean and a ratio lower is rich. While this sounds like it would be ideal for performance it is NOT. Would be great for maximum economy though. Even your O2 equipped fuel efficient automobile that tries to maintain a 14.7:1 ratio when cruising, bypasses O2 sensor readings when under full load acceleration going into what they call open loop mode allowing the ratio to go richer for better performance instead of closed loop operation for economy.

For best power under fully loaded acceleration your A/F ratio should be down around 12.5 to 1 A/F ratio, plus or minus a few 10ths. (engine vary a little) The flatter or more consistent that ratio remains from a low RPM to Redline while accelerating hard the better your performance will be. This has been dyno proven on all internal combustion engines countless number of times. Not just my opinion. When cruising or lightly loaded your A/F ratio will be leaner (Automatically based upon the stock EFI sensors) but should never go above 14.7:1 for any appreciable amount of time unless decelerating or engine braking. You can usually ignore A/F ratio readings when cruising once you are sure you are not going too lean. Myself I like to see the A/F ratios between 13.5-14.5:1 ratios during these lower load riding conditions. It will vary a lot and it is normal and desirable for it to do so once your have Full loaded acceleration set to around 12.5-12.8:1 ratio.

Cold Engine Idle A/F ratios should be down around 11.0-11.5 to 1 for best cold weather engine starting. Any leaner then this is difficult to keep the engine running when you first start it up cold in the cold. This value is at the verge of fouling the plug and probably would with extended idling.

The Electronic Acceleration Pump feature of the PC-III or on the PC-V devices is great, you can tune out that initial lean lag when you first snap open the throttle. I don't think any other manufacturer of fuel controllers have this feature. ?????

I use an Innovate LM-1 wide band O2 monitor to test my actual A/F ratio under various conditions, RPM and amount of throttle opening on my Tcat. With that information I can modify the fuel tables in the Fuel controller to optimize for best power or better economy or a balance in between. Once I get the fuel tables set to where I want them I remove the sensor replace with a bung plug and put the tester away until I suspect a problem or just want to retest. On my Road bike I have full time A/F measurement with a display in the center of handle bar.

The PC-V does have a self tune optional add on device which should work out pretty good for many. It does that by use of an O2 sensor that is installed into the exhaust system. Based upon what the self tuner reads from the O2 sensor it self modifies the fuel tables in the base unit as you ride. In theory it should work great. I have no personal experience with the PC-V or it's self tuner but several years of experience with the PC-III's on my Yamaha EFI Motorcycle and Tcat now.

There is a problem with all wide band O2 sensors on ATV's though, They hate water, if cold water reaches a hot sensor probe they are destroyed. Just like hitting a hot light bulb with a squirt gun. Instant destruction. There goes another $65-$85 out of your pocket. Although I would see no problem with installing the self tuner, let it self tune while avoiding deeper pits and once its done its job, remove the self tuner and O2 probe.

As it been said via actual A/F ratio measurement and personal experience of many, A Fuel controller may not be required with minor mods such as intake and performance mods. This is because the ratio will most likely never go lean enough to cause damage. I've confirmed this on both my motorcycle and on my Tcat with actual A/F measurements. However with Intake and exhaust mods you will be some performance boost with the mods only but you probably will not get maximum performance unless you install and tune A/F ratio for maximum performance.

If one were to put an engine on a dyno, First: Stock, Second: With intake and exhaust mods and no controller. Third: With intake and exhaust mods and with a controller you're going to get the best performance with the controller once it is tuned every time in my opinion.

Closing Comment
If in doubt, you always want to error towards being too rich. Being too lean can DESTROY your engine. Finally Please do not ask me what jets to use or if your jetting is correct. I cannot tell you this accurately setting here at my computer, I cannot hear your engine run, feel it's performance and I do not have experience with your specific intake and or exhaust modifications. I cannot closely inspect or read your plug from here either. I've already said a plug picture is probably garbage when it comes to reading plugs. The bulk of my experience in jetting was on motorcycles, Go-arts, snow machines & automotive, also with my 650H1 and a 400 manual Cats. Although I feel I can jet nearly any carburetor hands on, I cannot recommend jetting changes for a specific engine or for any set of specific modifications you may of added from a distance. As I said above, there are just too many variables.

I now use an "Innovate LM-1 Wide Band O2 Sensor" I got to say it's the best jetting tool I've ever used. What used to take all day or several days, now takes minutes to a couple hours at most. I highly recommend wide band O2 analyzers. No plug chops and No plug reading required. Just make a few runs at various throttle openings, read the A/F ratio and jet accordingly to achieve the desired A/F ratio. Wide Band O2 readers are available from a number of sources and different makes or models. They range in price from 200-500 dollars. The link below is a great package for a durable full time installation and what I use on my motorcycle. The same setup should work well on any ATV too, carburetor or fuel injected.

The same link above also has a dry nitrous universal NoX kit that adds up to 40 HP to a FI engine for about $209.00. I'm thinking it would work well on the 700H1 or the Tcat. For those that compete in sled pulls and mud bogs it may be interesting. Wet systems (More power) are also available for more $.

If the above isn't enough to satisfy your craving for power, there are always Superchargers and Turbos that you could install.

As I think about things to add to this post, I'll return to edit it, so it may change in time. Be sure to check for any updates from time to time. What you see here as evolved over the last several months. If you have corrections, disagree, have suggestions, what to add, remove or correct here. Let me know in a PM and we'll get things right.

Good luck and be careful Ride safe and Keep rubber side down.


Premium Member
5,047 Posts
Since you sound pretty sharp, I need an opinion. I put the full speedwerks kit in my 400, clutch, pipe, K&N. Since I ride at about 7000-8500, I also drilled out the top of the airbox.I also did both spring clip and slide drill mods. I have a 135 main from a stock 142, I may need to go up a size more. I went from a 38 pilot to a 40. I have a bad hesitation from off idle when I nail it, then it takes off. Im thinking I may need to richen the pilot even more, but Im not sure. The mixture screw is about 1 3/4 turns out. It acts the same in all temps. What do you think? Thanks, Frank

Premium Member
5,047 Posts
I love the supercharger part!! I should have thought about the tape or open airbox thing. My old XR600 had a stumble off idle, and I finally drilled a pilot out to richen it enough. But this isnt a stumble, it just hesitates and doesnt do anything for a second or so, and then takes off. You may be right about the spring clipping. One problem is, my wife rode it most of the day we bought it, then I brought it home and immediately took it apart and put the kit in, so I dont even know if it had a hesitation before all the mods. I will play with the air flow and see if I can get an idea od what I need to do. From all the other bikes Ive played with, I think I will go a bit higher on the main also. It runs good now, but I want to fine tune it. Thanks for the feedback!!

73 Posts
Wow great post and info Wyo. Very interesting plug chopping guide too. Its different than what I've seen before in that it says to check the base ring instead of the electrode for jetting. I think that does make more sense though from what I've seen with my cat though. It seems like my electrodes never really want to change any color. I'm not sure about running the engine at constant throttle for 10 minutes though, idk about you guys but I don't have any place where I can ride full throttle for more than a minute or two without dieing.

73 Posts
Yea there is definately a lot of places to ride here. I have a few hundred miles of trails within a half hour drive from home. Theres no mud like in MN but I think that riding in the mountains is more fun.

As far as riding at a constant trottle for plug chopping, I wasn't concerned about doing damage to the engine just doing damage to my self when I fly off a cliff or something trying to ride full trottle for that long. I don't think I've seen a trail yet where I could do that here. I might have to try to find a lot quiet gravel road.

157 Posts
Wyo H1 Cat (Future Wyo H3 Cat?) this question is a general altitude question. Is air-jetting a possible solution for riding in higher altitudes? Basically if I have a 1 1/2" intake hose and am set-up for sea level and plan to ride for a week at 6500 ft (actually will vary from 6000 ft to 11000 ft) then could I just have an additional air intake hose that would allow more air in the filter/carb? Kinda like using a 1.5" intake hose normally and using a 2" at the higher altitude?

This seems to make sense to me since at higher altitudes it just needs more air. Seems like I could install several 3/8" hoses and unplug in order as it seems to run rougher. The H1 has a perfect place for this in the lid right above where the intake hose connects to the airbox.

Air Jetting:

157 Posts
Looking at the Arctic Cat parts diagram for the H1 carb am I reading correct that the stock main jet is a 138? Looks like if the temp stays constant that I would be needing a 130 jet for the 6% difference (Sea Level 68F vs 6500ft 68F).

If the jet difference for a 2" intake vs a 1.5" is approx 6% then the air-jetting should work without issue. If not then I should likely rejet.

6,867 Posts
Discussion Starter · #8 · (Edited)
<div class='quotetop'>QUOTE </div>
Looking at the Arctic Cat parts diagram for the H1 carb am I reading correct that the stock main jet is a 138? Looks like if the temp stays constant that I would be needing a 130 jet for the 6% difference (Sea Level 68F vs 6500ft 68F).[/b]
Yes you are reading the chart correctly, 138X0.945 = 130.5 This would probably be rounded up to a 132.5 if I were to re-jet a flat slide or round slide carburetor. I'd use a 130 only if I knew the temperature would likely remain stable or increase. I always round up on the calculations.

On the CV carburetors like the cat ATV's use I'd probably drop another 2% from the chart recommendation, and end up with 138 X 0.965 = 133.2 and probably round up to a 134, 135 main or what ever the next available size is above the calculated value. CV carburetors tend to self compensate for altitude changes at least one full jet size +/- over a non CV carburetor because of the effect atmospheric pressures has exerting pressure upon the slide diaphragm.

<div class='quotetop'>QUOTE </div>
If the jet difference for a 2" intake vs a 1.5" is approx 6% then the air-jetting should work without issue. If not then I should likely rejet.[/b]
HUH? How did you determine that? The jetting chart is not for and cannot determine that there is a 6% in fuel requirements between 2" intake and 1.5" intake.
Cross sectional area difference between 2" and 1.5" is a lot more then 6%. The cross sectional area of 1.5" intake is about 1.8 sq inches, 2" cross sectional area is about 3.1 sq inches or nearly 75% larger in area.

Volume of air non turbulent flow through a 2" ID smooth pipe 1 ft long at 14.7 psi is about 385 cubic ft a minute. 1.5" ID drops flow down to 215 cubic ft per minute, This is about 80% greater flow volume with the 2" Expecting beneficial results with air jetting is difficult because of the Fuel to air ratio. At 13:1 Ratio that means you are burning 13 lbs of air for every 1 lb of fuel, It takes one hell of a lot of air to weigh a pound.

Just because the cross sectional area is almost 75% and the flow is about 80% larger it doesn't mean you're getting 80% more air into your engine. If you were to get 80% more air into an engine the difference of fuel required would be more then the 6% fuel change.

Air jetting or adding additional intake area or less restrictive intakes will only work if your present intake is already starving the engine upstream of the carburetor for the air it needs. As you increase in altitude, the air gets thinner (less oxygen per cubic ft) and air pressure drops in PSIA. These two factors are why you require less fuel. Adding additional air intake volume if you are already have enough cannot raise the amount of oxygen per cubic ft and cannot raise the atmospheric PSIA air pressure.

These links have some pretty interesting calculators.
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6,867 Posts
Discussion Starter · #9 · (Edited)
speed jetting and O2 measurement

If you located a O2 sensor and display for $30 then i guarantee it is a narrow band o2 sensor, not a wide band sensor. A narrow band sensor will not work for you because their range of operation is too narrow and accurate only close to 14.7:1 air fuel ratio regardless of the claims made by the ad or supplier.

A wide band sensor works and is accurate from A/F ratios from 10:1 to 21:1 and to get the best power you'll need to adjust to a 12.5:1 ratio +/- a few tenths. For good cold starting the ratio should be about 11.5:1 at idle but go up to 12.5:1 at steady mid throttle & full throttle UNDER LOAD. A/F ratios will lean out over 13 maybe even closer to 14:1 when the engine is lightly loaded or unloaded. That is normal and desirable. To load the engine, you need to be going uphill or read it quick while it is accelerating only to the desired throttle position. 10-15% grade should do it.

The cheapest way to get wide band measurement I've seen is with the LC-1 and a simple DC voltmeter, 1.00 volt output equals 10:1, 2.10 volts out equals 21:1 ratio. I've seen used LC-1 on ebay to around a $100 a few times they normally sell for $199. A wide band sensor alone is normally 70-90 dollars and that is without the electronics to read and control it.

Read the section covering plug reading in first post of this thread "jetting 101" and look for that thin layer of carbon buildup on the base ring of the plug. If absent you're lean, if fluffy you're rich. Don't worry so much about color on the ceramic around the electrode on short duration plug reads. However there will be some color down inside the plug and will require bright light and a magnifying glass to see it. There will be no color or very very little color on the ceramic that is exposed near the electrode tip. Unleaded gas is the main reason its now much harder to read plugs then it used to be. MeOH other gasoline additives make colors lighter using today's gasoline. It was the lead in gasoline that created the nice cardboard brown on the ceramic when you were jetted correctly.

Speed jetting for setting main jet size may also work well for you, use either elapsed time or top speed over a set distance and keep increasing main jet size as long as either time or speed improves but stop and go back one jet size when it decreases or stop when improvement remains constant. The distance or time should be long enough for the engine to be pulling hard while accelerating from a stop and pinning the throttle wide open. Speed jetting Must be done when barometric pressures and temperatures are similar for all speed jet testing though. To jet the needle height, speed jetting will not work as well, You'll have to go by feel and increase needle height as long as the acceleration seems to improve from a dead stop to approximately 3/4 open throttle. You can set throttle stop screw to prevent the carburetor butterfly thus the slide from fully opening. This insures you are staying on the needle circuit where the needle height is regulating the flow of gas into the engine. Look for best performance with out hesitation or stumbling. you can either use a dynajet adjustable needle or install shims under a stock needle to raise it. #4 machine screw washers make great needle shims.
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Discussion Starter · #10 · (Edited)
Quote: Originally Posted by doorfx.
Can someone explain to me the difference between an exhaust gas analyzer and a wideband O2 sensor? I see the O2 sensors on e-bay with a fitting that allows you to get a reading at the tailpipe,does this work??
A full blown exhaust analyzer may read and report the amount of Un-burned hydrocarbons, and amounts of CO2, CO1, O2, NOX in the exhaust stream. Called a 5 gas analyzer. A O2 sensor only reports the amount of O2 left in the exhaust gases that is in direct relation to the A/F ratio of the mixture being burned. Because of this, O2 sensing equipment rarely reads out the amount of oxygen in the exhaust gas and instead it displays the calculated A/F ratio.

There are two type of O2 sensors, Wide Band and Narrow Band sensors, a narrow band works more like a on/off switch telling the on board computer that the resulting mixture is either richer than 14.7:1 or leaner than 14.7:1 with very little resolution of actual A/F ratio and works or display properly between 14.2:1 and 15:1 ratios only. Above or below these ratios it is completely inaccurate. Therefore as a tuning tool, Narrow Band sensors are garbage! There are devices you can purchase that consists of a narrow band sensor and a display that are equally garbage as a tuning tool. Read the fine print.

Most automobile EFI systems use narrow band 02 measurement only when cruising and under light engine loads or go into a closed loop mode to save fuel and reduce emissions. When you stomp on the accelerator, the on-board computer usually ignores the narrow band output and goes into a open loop mode allowing for a richer A/F for more power. Your typical automobile also has a lot more sensors and are better equipped to maintain closer to ideal mixture settings under all conditions. Guess you could say the Automotive EFI systems are smarter then an a Cat ATV or most motorcycle EFI systems are.

A wide band is accurate and linear usually from ratios from 10:1 to 20:1 depending upon make and model and much better suited as a tuning tool. The wide band sensor is usually located at least 12" from the exhaust outlet somewhere in the exhaust piping all the way to the exhaust outlet. Some suppliers have probes or equipment that you just attach to the tail pipe outlet and I've seen reports where that does work. Only requirement for Wide band sensors is they hate liquid water the element shatters like a light bulb getting hit with a squirt gun. If they are installed too close to the cylinder head they can get too hot and read inaccurately. And if any external air oxygen from the atmosphere in introduced to the sensor It will be inaccurate.

Under full engine load acceleration you'll get your maximum power at ratios ranging from 12.0:1 to maybe 13:1 ratio, (Depends upon engine type, gasoline octane & additives) or just consider 12.5:1 ratio is good enough. Unless on a Dynomometer, doubt if you can feel the difference between a 12.2, 12.5 and 12.8 ratio although the difference may show up on a dyno.

You'll get your best economy when the ratio is higher and between 14:7 to 15.5:1 ratios. 15.0:1 and above is considered a lean-burn and unless your engine is specifically designed for lean burn it should not have a cruising light load A/F ratio leaner than 14.7 to 1. An engine that is experiencing lean burn and not designed for it can be damaged. Arctic Cat engines are Not lean burn engines.

If you adjust carburetor jetting or EFI fuel mapping to give you 12.5 +/- .2 or .3 ratio while under full load engine acceleration and keep it flat and consistent throughout the acceleration band you'll get the best performance. It is normal and desirable for the ratio to lean out at lighter loads above 13.5 to maybe 14.5 ratios and while decelerating it may go much higher or leaner and be perfectly normal. If the ratio goes too lean on deceleration you'll get loud explosive backfiring. I tune my carburetors and EFI mapping for a flat as possible 12.5-12.8 to 1 ratio with the throttle wide open while under load from the time I punch the throttle open until the rev limiter is reached. If you can achieve ratios like this while re-jetting or remapping a EFI controller, call it good. Everything else should fall into line. While cruising I hope to never see a ratio any higher then around 14.5:1. For cold weather starting I like to see the ratio down around 11:1 when the engine is cold and first started and idling.

There is a lot of information on the internet that goes into much greater detail then this and anyone that is going to attempt to tune EFI or Rejet for power should take advantage of the information available if one uses O2 measurement as a tuning tool. Remapping EFI is a little more involved and a throughout knowledge of both the remapping software and fuel controlling hardware used would be required along with some EFI basics. It will take some time to do

Generally, the EFI system on a stock ATV is pretty good at maintaining a desirable A/F. There is very little if anything to be gained by installing a fuel controller on a EFI'd ATV without mods. Any increase due to installing a fuel controller only may show improvement on a dyno run only. You may be unable to feel a difference. Only when there are modifications to the intake, exhaust or internal engine mods are performed is a fuel controller any benefit. An exception may be a replacement ECU or a device that allows one to reprogram the ignition timing curves. Dyna-tek is the only aftermarket provider that I've seen that has this. There may be others as well. Advancing the ignition timing where it reaches full advance sooner and advances further usually results in a performance boost.

Often minor mods in intake and exhaust may not require a fuel controller and installing a fuel controller may only make a small difference. There is a large number of users who have installed snorkel systems, added aftermarket exhaust and have not installed a fuel controller and they are having no problems and they got a performance boost from the minor mods. Will everyone experience the same result? I have no idea. Will everyone not require or require a fuel controller? I have no idea. A fuel controller is not the magic bullet everyone thinks it may be. Because the stock EFI system seems to be able to self adjust in a positive manner for some minor mods without a controller. Disclaimer, Yes there are or maybe times minor mods in intake and exhaust may definitely require a fuel controller. Testing the resulting A/F ratio is the only way to know for sure. I fully recommend that you know for sure. Get it tested.

When major mods are performed on a EFI system, a fuel controller will definitely be required. And the user should be well versed in understanding their EFI system, the controller, and the software used to reprogram the controller or leave it up to the experts. Although I feel confident in making adjustments on my PCIII fuel controllers I have on my Tcat and my motorcycle at my altitude. I have not installed any major mods on my tcat or motorcycle so I've no experience dealing with those issues with on an motor toy EFI system. I have zero knowledge in any other types of fuel controllers or on other applications, therefore I do not consider myself an expert by any means. I from time to time share what I've learned, however I am not of that much help really. <grin>

6,867 Posts
Discussion Starter · #11 · (Edited)
You Don't jet EFI. So why is it here in jetting 101? Because there is no need to get a separate EFI help thread pinned this way, <grin>

Lots of people do not have any idea on how EFI works and that makes trouble shooting much more difficult. Although some of the links below are specifically geared to Harley Davidson EFI systems, they are a very good guide to understanding Arctic Cat EFI systems. They work the same way.

Understanding your EFI. The basics.
Harley-Davidson Motorcycle Fuel Injection Explained

Tuning EFI with wide Band O2 measurement.
Portable Tuning with the Innovate LM-1

Sampling the exhaust stream with O2 probe.
DIY Motorcycle Exhaust O2 Sensor Probe

Modifying EFI options.
Harley-Davidson Fuel Injection Upgrades

A do it your self Fuel controller. A Poor Boy option that works and discussion of some of the devices out there that make outrageous claims to stay away from.

Do you really need a Fuel Controller?
If you want the best performance your engine can deliver then the answer is Yes. Even on a stock machine. The fuel mapping on a stock EFI controller is not set to optimum and probably not delivering anywhere near optimum A/F ratios. For all other reasons then...The correct answer is MAYBE. The same answer applies for changes in exhaust and intake. Doesn't matter if you have done a muffler mod or added a $500 replacement muffler.

EFI systems on all nearly ATV are pretty simple. Another way of saying pretty stupid. I've not seen any that utilize a "Smarter" Closed loop system from the factory yet. ????? The ECU has no idea of the resulting Air/Fuel ratio is at any time under any condition. Couple things to keep in mind with this answer; Are we talking about developing maximum power or engine safety and not experiencing engine damage from being too lean? If you want maximum power then YES a CONTROLLER IS always REQUIRED. It's even required if you want the best power possible and everything is kept stock because the stock fuel mapping is rarely optimum for your ATV even stock and brand new. If you are concerned about is engine safety then the answer is Maybe. It's just might be close enough in fact it may be so close enough that a controller is not required if you can live with less than maximum benefit from your mods in the first place. . IF..... After performing mods to either the intake and/or the exhaust system and you are getting a drop in performance or and more backfiring or miss-firing them most likely you will need a EFI fuel controller to correct this.

Stored in the ECU is a look-up table that controls the injector pulse width for any combination of RPM and throttle position. This therefore controls the base amount of fuel injected. The fuel table is programmed into the ECU during manufacturing based upon some engineers displacement based slide rule calculations and initial performance testing. Amount of fuel injected is modified for altitude, intake air temperature and engine temperature. It is not capable to directly sense modified changes to intake and exhaust flow performance all. However there is some hope that the resulting A/F mixture will be modified in the correct direction following intake and exhaust mods. Why

Snorkel mods usually end up lowering the temperature of the intake air. This lower temperature will cause the ECU to compensate for the colder air delivery and increase the amount of fuel injected. Question is though will it be enough? MAYBE

Exhaust mods which increase the volumetric efficiency through an engine can develop more power which can vary the load curves sensed by the MAP sensor that is designed for altitude changes. Will these changes in sensed load be in the right direction and can this increase the amount of fuel injected when the ATV is under hard acceleration. Doubtful. Be satisfied knowing that the load sensed by the map sensor is different and has changed from what it was originally sensing. Question is, will it be better? MAYBE. The more power a engine has it experiences less load placed upon it. An lightly loaded engine usually runs cooler, cooler engine temperatures sensed by the cylinder temperature sensor allow the ECU to sense a colder engine and cause the ECU to inject more fuel as well to compensate. This sensor is mainly used for cold engine start up though. But I suspect it does have some affect during normal operation also. I don't know for sure. If it does have a positive effect, Will it be enough? MAYBE Lean engines often run Hotter, Does a typical ECU sense this and increase fuel to cool an engine? This I don't know. In theory it should I think. There are a lot of secrets hidden inside that black box that are not published. And I've never yet thought of a way to test this theory easily in real world operation and with actual A/F measurements.

So the answer is still MAYBE. On my Tcat I had a dual exhaust system installed and modified cold air intake. Although the A/F ratio was a quite a bit leaner than ideal and it was not dangerously lean as determined by a wide band O2 sensor under various conditions during my A/F ratio testing. It was WAY too lean to develop maximum power though. To get full benefit from the mods, a fuel controller in my case should be installed and needed. Even though Not needed for safety of the engine. I do not have access to a dyno however just from feel I could tell I had better performance after the mods with no fuel controller installed compared to stock and once I installed a fuel controller the performance increased even more by feel alone.

So the bottom line on my 2008 Tcat, with my mods at my location, No, a fuel controller was not required for the safety of the engine. But a fuel controller is required to develop maximum power. This may be true for the majority also. MAYBE The only way to know for sure is to get actual analysis of A/F ratio.

Companies that build fuel controllers will post recommendations (fuel maps) to use for various combination of intake and exhaust mods for the different applications for a particular ATV model and although very close, the result is not as perfect as they good could be for your machine. If you want that last 1/100 of a second to win a critical race you need a custom fuel map and matches your engine, you mods and your sensor inaccuracies. Doesn't matter if it 100% Stock or Modified.

6 Posts
Mr Wyo_H1_Cat,

I'm fixing to buy a EFI controller. Right now I'm thinking about the GEN 3.5. Do you anything about this brand. Its's going on a 2012 MudPro 700 with an HMF exhaust.
I look forward to your opinion!

Thank you Not worthy:

6,867 Posts
Discussion Starter · #13 · (Edited)
Mr Wyo_H1_Cat,

I'm fixing to buy a EFI controller. Right now I'm thinking about the GEN 3.5. Do you anything about this brand. Its's going on a 2012 MudPro 700 with an HMF exhaust.
I look forward to your opinion!

Thank you Not worthy:
I have no idea what a Gen 3.5 controller is or who makes it even. Unless it gives you the same tuning functionality and tuning capabilities as a Power Commander V offers, I'd personally avoid it.

EDIT: ok a performed a search for GEN 3.5 fuel controller and found this link. EFI - Electronic Fuel Injection Controllers - ATVs, UTVs, Motorcycles, Snowmobiles, Scooters, Sport Bikes, Watercraft. I see where it is capable of both adding and subtracting fuel from the stock fuel curves and it does have more adjustable ranges. That is a big plus, compared to OLDER Dobeck type devices which in my opinion were a $250 piece of crap. Is this Gen 3.5 version any good? I have no idea. Off hand it doesn't appear to have the same functionality and capabilities as the PCV. Maybe it would be easier to tune than the PCV. Don't know I'll never buy a gen 3.5 device to test myself anyhow. In my opinion... The PCV with the Autotune and the Ignition Tune options is still the ultimate device.

There are two versions of the Power Commander V , one with ignition tuning capability, one without. Both PC-V's can do closed loop fuel map programming with an add-on module based upon feedback from an O2 sensor to automatically build a custom fuel remapping table to suit your modifications on your ATV as you ride. The ignition tuning version of the PCV allows ignition table editing. Meaning reprogramming of the ignition advance curves. The Rev Limiter function in a stock ECU is performed by retarding the ignition advance when the max RPM is reached. Since you can reprogram ignition advance you can raise the rev limiter too. On a 08 Thundercat the ignition advance retardation begins at 7000 RPM. So at any RPM above 7000, the power drops off quicly as RPM is increased above 7000. Is it the same on later 1000's I have no idea.

Combined with fuel table remapping and ignition advance tuning to set advance a little further and advance sooner does wake up the 1000 engine quite a bit. You can Definitely feel a difference. Delivers a very Nice grin factor. To get all this functionally can be considered pricey though.

I've got the PC-V with the Ignition advance capabilities built in and I really like it. Maybe some day I'll add the "Auto-tune" feature to it. I would recommend it for the 1000's over any other device out there that I know of. Some are just junk in my opinion and some just don't work as they should. How would the new PC-V work with other cat engines if available for you model? I have no idea. I'm thinking it would work as well on a 700H1 as the highly recommend VDI device does for the Suzuki based 700EFI ATV's since the functionality is basically the same between the two. The exception to that claim is, the stock injector on the 700EFI is undersized. It was found that replacing the stock injector with a motorcycle M109 injector is an added boost. One that may not be applicable to the H1. Is the 700H1 injector adequate? I have no idea. I've studied the capabilities of the VDI device when there was discussion that they would create a H1 version someday. Based upon that alone I don't believe there is a single function that the VDI provides that isn't available with the new PC-V with ignition tune capabilities.

6,867 Posts
Discussion Starter · #14 · (Edited)
Cleaned and check everything in the carb, still bad?

This is a copy of text from a recent response to a thread in the main trouble shooting section. I think I've added a few tips there that are not in the first jetting 101 thread. I appended the text here.

HELP I've tried Everything and it is still not right, should i buy a new carburetor?

The last thing I would do is purchase a new carburetor. Rarely is that extreme needed unless the carburetor body gets damaged. In my time working on many dozens of Keihin carbs a replacement carb was never needed. NEVER

Have your determined where in the throttle it is running rich, Idle? Mid throttle? Full throttle? there are 4 distinct areas, the PMS for idle, the Pilot (slow jet) for idle and up to 1/4 open throttle, the needle for 1/4 to almost wide open the the main jet for wide open. There is some interaction between jetting circuits and the amount of fuel each jet circuit contributes however adjusting the PMS has very little influence upon overall fuel requirements at throttle openings greater than 1/4 open and likewise the main jet has almost zero influence at idle. Plug chops or and A/F ratio measurement will determine where your various jetting circuits is good or bad. See jetting 101 in the pinned section for further discussion on this. How the circuits interact and supply fuel and additional trouble shooting tips and how to correctly perform a plug chop at various throttle openings.

It is quite possible that the new stock jet are larger than it needs on the main jet. Why? A worn engine that has less compression needs less fuel. And even a new engine may be jetted richer than it should be even when brand new direct out of the factory. Why tolerances in manufacturing vary somewhat and the factory trends to jet richer than ideal during assembly.

That should have Keihin jets in it from the factory. Many install jet jets from Dynajet, Another company who makes jetting kits. The problem is a 140 keihin jet and a 140 Dynajet are NOT the same size. The bore in a dynajet is quite a bit larger and would require a smaller numbered jet to have equivalent fuel flow. If you are replacing suspect jets with new jets using a dynajet and following the service manual for the correct size WILL not work. I've also seen jets that were number stamped wrong too, Rare but it can and does happen.

A plugged air cleaner or plugged exhaust will also cause excessive richness. Weak spark or just a small amount burning oil leaking past the rings can cause the appearance of being too rich as well.

Another possibility is the fuel float level is set too high. Does your float Float? Plastic floats can be cracked, Brass float can corrode a pinhole and allow fuel to leak inside. Styrofoam floats can absorb fuel in between pores of the Styrofoam, in these three cases the float will be heavier and result in a higher fuel level in the float bowl causing higher fuel level and an over-rich condition. I've soldered up brass floats and applied super glue to plastic floats to seal them after getting all the fuel out of the cavity. Will this work? Yes it can. Keep in mind, repairing a bad float makes the float a little more heavy and may require setting the float fuel recommended height taller to shut off the fuel sooner coming through the float needle valve. Superglue may only be a temporary repair until a replacement can be ordered or found in my opinion though, since the fuel may destroy the superglue repair in time.

Altitude affects jet sizing. Higher altitudes require smaller jetting. In Illinois that may not be a problem However 2500-3000 ft above sea level may require the next size smaller. At my altitude I'd jet almost 10% smalle, 2 maybe 3 jet sizes smaller than my engine would require at sea level. All factory recommendations are for sea level

There is an actual needle jet too, it is the small stainless steel sleeve that the actual needle tip slides into as it travels up and down. That piece can get lost during diss-assembly and cause problems like this. you should be able to see the needle jet stick up a bit through the main bore of the carburetor as you lift the slide with a finger tip, IF all you see is a hole in the bottom of the bore, Your needle jet is GONE.

An ATV with a whole lot of hours on it can wear the actual needle to a smaller diameter or wear the needle jet to a larger diameter and that will cause excessive richness. This is rarely the case unless you have a lot of time and miles on an engine, probably greater than 5000 miles or more.

My recommendation to test a few things out is to take an old jet and solder the hole it shut completely. Install this plugged jet in the start jet location. This will totally disable the enrichment valve 100% and determine once and for all if the choke plunger is leaking by even though you installed a new choke plunger. It may be difficult to start cold with the choke completely disabled but it will rule out enrichment (choke) problems if it cures the richness. Also does it still run rich with the lid off the air box? (test in a clean environment only though, you don't want to suck unfiltered dirty air into you engine.)

Are you burning old stale fuel? A lot of volatile light compounds evaporate from fuel during storage. Old fuel does not burn as clean as fresh fuel and can result in black smoke at the tail pipe. If the fuel in your tanks is been in storage longer than a couple months it could be stale. Goes stale quicker with high temperatures too.

In a low compression engine Premium fuel doesn't burn as well either. Don't confuse the word Premium with Quality either. There are low quality premium fuels at some stations and at your cut rate stations. Arctic Cat recommend I believe 87 octane fuel. Unless You've increase compression with a high compression piston there is no need to use high octane. At high altitudes 83 or 85 octane is just as good due to the lower atmospheric pressures, burning higher octane than when your engine compression needs offers Zero Benefit. IF you are one of those few who claim you get better performance with high octane then you do with the recommended 87 octane, I'll bet you are not jetted as good as you should be, High octane burns slower that all, and in a low compression engine without additional ignition timing advance, a lot of unburnt fuel goes out the exhaust pipe. If you actually run better, chance are you are jetted richer than you should be, Then again for others, It just feels better spending the extra 5-10 cents per gallon in the belief you are purchasing a higher quality fuel. If I added any benefit at all I'd buy it. I've done enough comparison testing with wide band O2 measurement to know I can get great A/F ratios and better performance (Elapsed time/distance test) when properly jetted with 85 octane then I do with 91 or 93 octane at this elevation. Simple O2 measurement doesn't show unburnt hydrocarbon either. A 5 gas exhaust analyzer would be needed for that. and they cost more money than I'd be willing to spend. Now if you use a ignition programer to advance timing or high compression piston then premium or a higher octane fuel is probably required to prevent detonation. That is all premium fuels does, prevent detonation, ping, or knock what ever you wish to call it.

Have you added a lot of fuel additive? too much additive can result in smoke and even the proper amount of additive can clean or burn off built up carbon from the cylinder head, piston and valves causing black smoke for a while.

Make sure the air flow into the air box and the air filter are completely unobstructed. There is always a possibility of a mouse nest built in the intake duct work during storage that could obstruct airflow and cause richness.

Make sure the spark arrestor screen in the muffler is clean if that is serviceable. I'm thinking the 2005 is when they changed designs of the muffler the newer mufflers have serviceable spark arrestors

I'd closely inspect the needle and the needle jet for wear, make sure the needle taper is round and uniform and the needle jet is first installed and the bore is round.

Make sure the slide moves freely and the rubber diaphragm is in good shape (no pinholes) and the diaphragm is not leaking where it seals between the carburetor body and the diaphragm cap.

Make sure the tiny holes on the side of the needle jet retainer are clear (the needle jet retainer is that long brass piece the main jet screws into) and make sure every port and passageway in the carburetor body are clean. Especially those tiny air bleed ports located around the bottom third of the main carburetor bore air intake. This must be clean so the carburetor can correctly atomize the fuel into a mist

I've also refer to the service manual for the float height requirement and make sure it is set correctly. Most likely the recommendation is 17mm but yours may be different for the 05 500.

Another thing to check. Did you buy this new? Is the carburetor sized correctly? A previous owner may of installed the wrong carburetor at some point?

And it would not hurt to check valve clearance either.

Make sure the spark across the plug gap is a strong spark (bright blue) on a new plug.

1 Posts
i did a exh mod slide mod and dyno jet kit bike runs great but i get the low is to lean what jets should i start out with and where can i get the screw driver for the air pilot screw its a d shape bike is a 2009 500 manuel

6,867 Posts
Discussion Starter · #16 ·
i did a exh mod slide mod and dyno jet kit bike runs great but i get the low is to lean what jets should i start out with and where can i get the screw driver for the air pilot screw its a d shape bike is a 2009 500 manuel
Do a web search for Keihin Tamperproof screw carburetor adjustment tool, many suppliers sell them, Another idea is to replace that screw with a replacement that has a thumb knob on it. Doorfx a member here has posted the part number and source several times.

Or to get the screw out and to adjust it you can sometimes find a piece of round tubing that just fits and smash one side of it into a D shape. Sometimes needle nose pliers work too just to remove it for replacement

40,355 Posts
This is what the d shape tool looks like

This is what the e-z-adjust pms looks like

You can get it Canada through parts Canada and comes from Yost Performance
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