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Old 12-25-2016, 05:16 PM   #1
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94 ZR580 - Hard starts, low-mid range bog, poor top speed & RPM

Merry Christmas everyone!

After the kids and my wife went down for a nap, I finally got a chance to try out the carb'd 1994 ZR 580 that I had been fixing up over the last few months after buying it for cheap. The biggest issue that I was trying to figure out previously was very hard starting, which I found seemed to be due to flooding after a few days of sitting (I pumped a lot of fuel out of the crankcase). I replaced the needles and seats, and I close the fuel shutoff valve when not using it, but it was still difficult to start today after leaving it for the last couple of weeks, taking about 30 or more pulls to get it started. All safety switches, including TSS, have been adjusted and verified functional. There also has been more oil ending up on the ground/floor while warming it up than I expected to see.

Anyway, before I dive in further, I noticed other issues while riding it today that may or may not be related. I'm trying to make a list of things to address over the next few weeks while I'm not working.

1) There is a significant low to mid-range bog if I push the throttle hard. For example, if I'm running at around 20 mph and go WOT or close to it, I can hear the bog and it won't accelerate beyond 25-30 mph. If I feather the throttle and get to around 35 mph and then hit the full throttle, it pulls hard as I would expect.
2) Although I didn't have a long place to run it out (I could hold it WOT for about 20 seconds), I couldn't get it above 55 mph according to the speedometer. It would get to around 45 easily, then struggle for every mph after that. I would have expected it to hit 60 fairly easily. I don't recall exactly, but I don't think it was getting much, if at all, above 7000 RPM.
3) Right after cold starting, the idle is quite high, around 3000 RPM. If I pop on the choke, it drops down, but climbs back up until it's warmed up if I return the choke. After warming up, it sits around 1200 RPM. I couldn't get it to idle higher for some reason, even with adjusting the idle speed screw. The pilot air screws had to be screwed in almost all the way to stop a hanging idle after blipping the throttle, even when warm.
4) At 1200 RPM idle, the headlights flicker a small amount. I believe that this might just be the low idle speed not allowing for a consistent enough supply as it is fine above 1500 RPM.

Some things that I have already done:

-Thoroughly cleaned both carbs
-Replaced the main jets with new 370s (what the jet chart shows for my temp and elevation)
-Checked compression - 120 psi on both cylinders
-Piston skirts are intact
-Stator resistance tests showed that it seemed to be ok when cold
-Replaced coil, wires, and plugs
-Checked for crank seal leaks with carb cleaner and an unlit propane torch, but no change in idle speed
-Synchronized the carbs with homemade liquid balancing setup
-Replaced the carb boots
-Set the injection oil pump to the correct position
-Set the depths for the choke plungers
-Adjusted float arm height
-Checked clutch springs to confirm that they're still stock
-Other stuff that wouldn't be related (e.g, shims in secondary, coolant temp sensor, brakes, etc.)

The next time I'm out, I'm going to do a proper plug test, and I will be checking the impulse line for fuel from the crank to the fuel pump in case there's a hold in the diaphragm. In the meantime, is there anything else that I can check? Could poor grounds cause a weak spark that would result in difficult starting, poor idle speed/combustion of oil, low-mid range bog, and reduced top speed? I haven't checked the slide needle position, but that's already on my list.

Thanks for any advice!
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Old 12-26-2016, 12:32 AM   #2
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Well, first, your carbs must be way off. Sound like they are dysfunctional according to your explanations.

Questions for you:

1) Do you have a service manual for your sled? If not, I'll link one for you to download for free. You are going to use it to fix your sled.

2) Have you been monitoring the coolant level (under the pressure cap, not in the bottle) to be sure that your water pump seal (and/or cylinders and heads gaskets) are not leaking?

Your water bottle vacuum gadget (as shown in your previous post) is not a useful tool for carb syncing (in fact it is guaranteed to prompt you to make a mess of any existing calibrations) for two reasons. Firstly, there is a very, very, VERY, serious hysteresis problem with that gadget. Secondly, it is not a direct measuring device (such as a vacuum gauge, for instance) it is a balance/equalization type of device. It can not isolate and measure. The capability of the gadget does not fit the task.

It appears that your motor has multiple problems, but they all could resolve down to being just two, if lucky.

With the motor cold, jack the front end of your sled up about 14 inches and check your coolant level under the pressure cap first (there should be NO AIR under the cap) and let us know if it is not completely full.

Then verify your fuel lines are all good, starting from inside the gas tank all the way out to the carbs. Replace any of them if they are in the least bit questionable. The pickup line inside the tank is bound to be rotted if its had any ethanol fuel left in the tank. Tell us what you found and what you replaced.

Don't forget to replace your fuel filter with OEM only, and check the condition of the fuel pump pulse hose along its entire length.

You need to set up your carbs again from scratch. Total disassembly is required... along with the service manual at hand for initial internal and external carb settings and proper cable adjustments and synchronization method.

Last edited by SevenBlizzardsRoad; 12-26-2016 at 01:11 AM. Reason: Happy new sledding year!
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Old 12-26-2016, 08:38 AM   #3
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Thanks for the response. Can you explain why the dual bottle vacuum differential gauge setup is not acceptable if adjusted when held at specific positions? From my reading, it seemed like it was the standard way of balancing them to ensure that both cylinders are working the same amount at idle and off-idle, and commercially available equivalent products are also available. Is the visual/by feel method as in the service manual more appropriate? It seemed like a less accurate way in lieu of a gauge, but I'm open to anyone's thoughts.

I do have the service manual and have used it a fair bit. This is my first sled and I've been learning from scratch from an automotive background.

In terms of the cooling system, I did a full drain and refill, and I believe all of the air is out. I checked the coolant level a number of times when testing things out and it seemed fine. There's no white smoke that would be indicative of burning coolant if that's what the concern is?

Anyway, I'm hoping to better understand how things might be interrelated. Please share your thoughts as I'm really hoping to get this thing running strong. Thanks.

Last edited by Aaron407; 12-26-2016 at 08:48 AM.
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Old 12-26-2016, 02:53 PM   #4
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The bottle gadget would be fine on the simplest type of device where only one or two basic types of action (& reaction) exists. In a simple environment where few dynamic properties and processes exist you can balance that environment, provided your bottle gadget is very responsive to small changes (appropriate hysteresis) as you adjust.

A twin cylinder motor is no such animal, because mechanical and thermodynamic (and other complex AND dynamic) differences exist between each functioning cylinder. In such a complex and dynamic environment the tuner must attempt to optimize each functioning cylinder, not balance them. Any balance/equalization properties that each cylinder exhibits have been provided for during the engineering and manufacturing stages, so it is impossible to alter while its running.... Again, in tuning such a critter you must optimize.

An example: lets take a twin cylinder motor where identical components make up each cylinder. There are still many measurable differences between them no matter how precise we engineer and build them though. Each are never perfect and each can not ever match performance with the other. As they age and wear those differences widen. Is a proper tuning approach here one of seeking a balance? Equalization of input/output for each? Think about the consequences in that question...

To tune that motor so that each cylinder performs identically to the other thermodynamically, means that the strongest performing cylinder will have to be detuned to match the weaker output of the other. You'll limit input (air/fuel or other means) to the stronger cylinder in order to bring it down to the same output as the weaker cylinder. This is providing balance. This is what the bottle gadget is capable of doing for you. You now have a motor that puts out less power than is possible though, than if your approach would have been to optimize each cylinder, rather than equalize them. Follow me...?

So, isolate, measure, optimize, is the tuners best approach. A simple vacuum gauge can be used by the tuner to do what you are attempting to do with the bottle gadget in a far more effective way. And there are limitless other tools and methods available to diagnose and tune a motor (or individual parts of it), some sophisticated and complex, some not at all, depending on your target. You can get a stocker in pretty good tune with just a few of the basic tools though.


Great that you have a service manual! Are your carbs the originals for your motor? Have you set the float levels of each carb to specs? Are your needle and seat assemblies sealing good by float pressure? Do all of your floats, FLOAT? Are all your passages verified to be clean and clear (plugged ports can be difficult to notice when cleaning) in each carb? Is your jetting correct in each carb (mains, pilots, needle jets, needles) for your elevation above/below sea level? You seem to have a flooding problem (hard to start, oil on the floor etc.) according to your explanation and your previous threads. Something may be wrong here with one or both of your carbs so the service manual spec'd setup is important to get 100% right.


The reason I asked so much about your cooling system integrity is because a leaking water pump seal (or base/head gaskets) can be a contributing cause for your oil puddles on the floor and leave you with a motor that is impossible to diagnose and tune.

This looks like a motor with multiple problems. My best advice is to start over from scratch, check everything from the tank to the carbs, set it all up according to specs initially, suspect everything and be thorough, since there are indications that one or more of your previous problems that you talked about in previous threads, still exist.

Great that you have some experience with autos... you'll find these 2 strokes to be a lot easier to work on than the govcorp jokes found underhood these days...

Have fun,

Last edited by SevenBlizzardsRoad; 12-26-2016 at 03:09 PM.
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Old 12-26-2016, 11:23 PM   #5
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Thanks a ton for the write-up! As an electrical engineer, you sound like a mechanical engineer to me...

I understand the balancing conundrum better now, especially the part about tuning to match the weaker cylinder and losing capability because of it.

Anyway, tonight I did a few things:

1) Resync'd the carbs per the service manual directions.
2) Readjusted the oil injection pump (it was set too high, which I'm sure was contributing to the high oil consumption).
3) Pulled the impulse like at the crank to check for fuel to determine whether there might be a diaphragm leak (everything looked dry).
4) Pulled the plugs, which were a dark brown color. Normally this would suggest a good mixture, but that might only be the case at certain throttle points and possibly not at the point where it bogs.
5) Tested the ground near the CDI, which seemed fine.

I fired it up, and managed to get a nice smooth 1500 RPM idle. After letting it warm up, I took it for a quick run, and the bog is definitely still there. Essentially, if I pin it from 20 mph, the RPM won't go beyond 5500, and the RPM and speed actually start to drop if I hold it wide open. If I run it past that speed by feathering the throttle and gradually go WOT, it will scream and accelerate how it's supposed to.

As for you other questions:

Are your carbs the originals for your motor?
-Yep, they appear to be.

Have you set the float levels of each carb to specs?
-Sure did.

Are your needle and seat assemblies sealing good by float pressure?
-They seem to be. I replaced them along with the small red gaskets when trying to fix the flooding issue.

Do all of your floats, FLOAT?
-Yeah. I actually pulled them out and maintained them submerged for awhile to ensure that there were no punctures or leaks, and they checked out. They're also sliding smoothly on the posts.

Are all your passages verified to be clean and clear (plugged ports can be difficult to notice when cleaning) in each carb?
-They seem to be. In addition to carb cleaner, I blew out the passages with compressed air.

Is your jetting correct in each carb (mains, pilots, needle jets, needles) for your elevation above/below sea level?
-I had put new main jets in, which match the elevation and temperature. I had pulled the pilots, but couldn't properly read the sizing. I believe they are still the originals, but maybe I should change them out just to rule them out. As for the slide needle, I haven't checked it out in much detail yet, but maybe that should be next on the list for me to read up on.

For the coolant, I popped off the cap tonight and a bit even spilled out, with the level right at the top. However, I noticed that the overflow tank level was a bit low. I'm not sure if there's some consumption happening as there may have still been a bit of air in the system being worked out from the coolant change. I'll keep an eye on it.

Anyway, although I haven't left it long enough to assess the starting ability after adjusting the carbs, I'm really curious about the bogging. Although I'm sure there could be multiple culprits, I'm thinking fueling. Should I be checking out the slide needle position next to see if it was messed with by a previous owner? However, could this even cause it to bog if it's happening at full throttle since tree needle is only supposed to affect mid throttle?

Thanks again for the help. I hadn't even driven a snowmobile before getting this thing and knew absolutely nothing about them before getting it, so I'm absorbing as fast as I can.

Last edited by Aaron407; 12-26-2016 at 11:27 PM.
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Old 12-27-2016, 12:54 AM   #6
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Not a credentialed engineer myself but broadly skilled in many, many fields and trades. Some quite high level, others having a more general understanding and associated skill set. Electronics design is one of my favorite hobbies... I fix all kinds of stuff, design and build things from scratch and modify almost everything interesting I can get my hands on. The TI website snags a bunch of my time and money... I can code well and have commercial software running a company top to bottom somewhere out there. I'm addicted to racing from a designer/engine builder viewpoint. My niche and love is Torque, how to get it in massive quantities from small-medium displacement conventionally aspirated motors fueled by methanol. I have dyno numbers that many refuse to believe is possible. Until they attempt to chase one of my motors down on a race track. Most of the time though, I don't exist.

Anyway, you are almost there!!! I'll try to find some photos to provide for understanding on some things to check on next. Be back as soon as I can... maybe tomorrow since its getting late?
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Old 12-27-2016, 10:43 PM   #7
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Alright, from what you describe it reads like there may be an issue in your fuel circuits controlled by the Needle jet and Jet needle in each carb. I know you have cleaned the carbs again and again, but play along here anyway because you should get back in there and check a few things before going off in another direction. I scrounged for images and found a few belonging to others (Thank you!), so I'm gonna clutter dump them here as I ramble on.

First is a generic image showing the various enrichment circuits in a Mikuni carb in relation to throttle opening percentage. This may help you to guess in which enrichment circuit(s) your issue may be while diagnosing a problem or while improving a tune.

The Needle jet in each carb is brass and should have a short portion of the jet body sticking up into the venturi (Make sure yours are not broken off, those cutaway protrusions are critical to the operation of the circuit) where passing air can lift and bust up the liquid fuel into tiny droplets, as the pressure within the venturi is lower than atmosheric, while the air velocity is high.

Fuel moves from the float bowl up through the Needle jet where a small amount of high pressure air is allowed to mix with the rising fuel column and then that mixture enters into the swift air stream in the venturi area of the carb, due to the pressure differential created by the increase in the speed of that air. The cutaway at the top of the Needle jet provides a secondary (booster) venturi low pressure area. Turbulent (mini vortex form along/behind each trailing edge of the booster), high velocity air rips the entering fuel column into a gazillion pieces and scatters it throughout the column of rushing air. Visualize a tornado in action along the ground...

As velocity goes up, pressure goes down, (and vice versa) its a physics rule thats always true with moving columns of gases within a closed system.

There are basically three points here within the Needle jet where we sometimes find trouble. The most common problem is the tiny holes in the tubular walls of the jet where fuel is being metered into the bore of the jet (and separately, air is also allowed to be introduced into that liquid fuel just above), become plugged. Sometimes its dirt or other junk that piggybacked in with the fuel causing the blockage. Sometimes it is old and decayed fuel that has broken down into a near solid state. Other times its corrosion caused by water and/or oxygenated fuel such as ethanol. Many times its a combination of these.

Alcohols are corrosive to metals because they bring their own oxygen along with them, so aluminum and other soft metal parts are under constant attack and given enough time, all can be reduced to gunky junk. If at all possible, put ethanol free fuel in your tank only. You'll have fewer breakdowns, save yourself so much grief and expense over the long term... in short, your time can be put toward being productive, rather than being wasted solving recurring problems that are certain to happen.

The second most common problem found in the Needle jet circuit is that the high pressure air supply (at or very near atmospheric pressure) passage becomes blocked. This passage begins just inside the carb throat area where the diameter of the throat is largest. If you take the boot off your airbox and look inside the carb throat you will see the small hole where air is introduced (branching off through separate internal passages) to the pilot jet, the air screw for idle mixture adjustment, and into the Needle jet where air is introduced as mentioned above. There is generally one entry hole needed in the carb throat for clean air, but inside the carb body that relatively large hole is branched off through much smaller drilled passages to supply both the Pilot and Needle circuits. Inside, where those smaller passages are drilled to meet the larger entry hole, is where the plugs of dirt, mouse nest debris etc., are usually found.

While you have the carb completely disassembled (nothing else can possibly be removed = completely disassembled!) for each cleaning session, always force compressed air or a carb cleaner solvent through those passages from each end of the six possible entrys/exits and verify that your compressed air or solvent makes it all the way through, coming out all the opposite ends. There should be a heavy flow through the Needle jet circuit and a lighter flow through the pilot circuit. This is especially important on carb models that have pressed fit Needle jets, as they cannot be easily removed for cleaning without damaging them. If you find a restricted/plugged Needle jet that you cannot blow open, it will have to be replaced.

The third most common problem with a Needle jet is breakage of the cutaway protrusion at the top of the jet (as mentioned above), and/or a high amount of wear caused by the Jet needle vibrating within the bore of the Needle jet over time, causing the motor to run rich during a portion of that jets' operating range. Piston port motors are generally more likely to cause Needle jet and Jet needle wear because of the strong induction standoff pulses inherent to that design.

If the cutaway portion at the top of the Needle jet is found to be broken off or missing, the fuel will begin its entry into the venturi air stream very late in response to increased throttle opening. The fuel will eventually gush into the venturi area once the pressure drops low enough, but most of that fuel will puddle up and flow along the floor of the carb like a river, never getting up into the turbulent column of air to become atomized by it. That little chunk of brass doesn't look important, but it is... you're not going anywhere without them.

The Jet needle is a tapered rod and little more... It plays a minor role of being a "fine tuner" of the Needle jet circuit. It plays a much larger role of controlling the rate of increase (mass) for gross fuel flow through the Needle jet, relative to throttle position. There are usually five (5) grooves cut around the top of the Needle jet where an e-clip is used to preset its depth in the bore. The most common e-clip position is #3, the center groove, so if you don't know where it should be due to lack of documentation, clip the center groove and fine tune it later.

Thats about all I got for those midrange circuits... aren't you glad I am about to shut up!?!?

If I am guessing wrong then you can flog me and move on to find the real problem. First, lets eliminate this probability though... Game on?

Tip: There are sometimes (many models have these) one or two tiny plastic washers above and/or below the e-clip on the Jet needle shaft. Watch for them to fall out and disappear under your motor as you take the throttle cable off each slide. Also beware, the little magnets that control the TSS switches can/will fall out too. Those lil' buggers can break into pieces if they drop onto something hard. Best practices include placing a dropcloth or something similar below your work area to catch all your loose parts as you disassemble and reassemble.
Attached Thumbnails
94 ZR580 - Hard starts, low-mid range bog, poor top speed & RPM-carb_jet_usage1.jpg   94 ZR580 - Hard starts, low-mid range bog, poor top speed & RPM-img_2055.jpg   94 ZR580 - Hard starts, low-mid range bog, poor top speed & RPM-njet.jpg  
Attached Files
File Type: pdf Mikuni Tuning Components.pdf (136.2 KB, 44 views)

Last edited by SevenBlizzardsRoad; 12-27-2016 at 11:25 PM.
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Old 12-28-2016, 02:59 AM   #8
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I should add for a fuller understanding...

The main jet circuit does not come into play at all until the throttle is both wide open, AND there are sufficient RPMs to bring it up. This requires that you get the motor "up on the pipe", so to speak. The RPMs must be very high because the main jet functions as a mass delivery device AND pretty much functions ALONE in supplying the motor with the proper amount of fuel at WOT and high RPMs.

The main jet is either fully supplying its maximum allowed amount of fuel to enter the throat or it is completely offline, supplying nothing. No venturi of sufficient size is available to assist it. It cannot function until a certain velocity/pressure differential within the air mass is reached and exists.

Lets figure out this "one-trick-pony", main jet circuit. When the main jet circuit comes on:


Well, the idle circuit is no longer functional because the extremely low pressure, low velocity environment (the high vacuum area between the closed throttle slide and the crankcase) required for it to work is gone!

Also, the venturi previously defined by the POSITION of the wedge shaped cutaway on the lower incoming edge of the throttle slide within the throat bore, that was available for low to part throttle openings to increase the air velocity (while simultaneously lowering the pressure for those lower speed circuits to function)... whew! that was a mouthful!... is gone!

At WOT the throttle slide has been pulled all of the way out of the top of the throat bore, so nothing having a sufficient size or shape remains in the bore to provide for (and manipulate) any possible air velocity and air pressure differentials. There remains only a raw, straight, venturi-less "pipe" for the incoming column of air to rush through. It is unrestricted, except for that little booster chunk of brass on top of the Needle jet scattering the incoming fuel into the airstream like a mini tornado.

So, all of those fuel delivery circuits that were previously working together (note how each circuit must overlap at least one other circuit at any given point in throttle opening) to bring the RPMs up sufficiently for the handoff to the main jet, are essentially dead at WOT! The handoff to the main jet occurs rapidly within a very narrow overlapping band as the midrange circuits are dying out.

A forum allows little more than an armchair quarterbacking exchange between users, so helping another member find a problem many times results in the person(s) trying to help... being dead wrong. If you feel that we (I) are on the wrong track at any time here, bail out.

From my armchair view it sounds like you have an idle circuit fully functional along with a low speed enrichment transition supplied by the throttle slide cutaway (the primary venturi)... both of them it sounds like to me, are fully functional according to your explanations. Correct me if you feel either of those circuits are lacking and explain....

If the above is true that leaves only the midrange circuits within your carbs that may still be nonfunctional, since you said earlier that you can get it up on the main jet (it rips and pulls hard right?) by feathering/coaxing the RPMs up using patient and careful throttle control.

So I am going with the play I jabbered on about in the previous huddle above.

Hope these rants help, rather than hurts!

Last edited by SevenBlizzardsRoad; 12-28-2016 at 03:21 AM.
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Old 12-28-2016, 09:43 AM   #9
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What was the main jet in it before?

370 sounds too rich, even for lower elevations. I find that what AC recommends for main jets for the ZR are too rich. When I was jetting mine, I started at 410 (way too rich) and I eventually settled on 320's and according to Google, my elevation is 305 feet (93 meters)

Also, what are your needles set at?

When I adjusted the needles on my VM40's I read on this forum that the recommended spot was the top notch, or #1 notch from the picture above. When I had them set anywhere else it was too rich in the mid range.

The pilot jet should #40. I would replace if you are not sure.

This video is for a CV carb, but illustrates very well how each jet works and what is happening at various engine speeds. Only difference is that our carbs don't have a butterfly valve and a vacuum operated slide; our manual slide replaces that. But what you can see from the video is that the main jet does come into play in all ranges, except idle.

Last...you are actually supposed to tune carbs from the top down. Meaning, you should get it working at WOT first. That way you know you have the correct main jet. Then get the mid range working - Needle setting. Then get just off idle - float level and pilot jet. Then idle.

And if in doubt about the fuel pump, rebuild or replace.

Just my 2 cents...
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580 Triple Port, VM40 carbs, 01 ZR skid, Polaris P85 primary with AC Orange spring and 49.5 g weights, roller secondary with AC yellow spring in middle hole and 53* helix

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Old 12-28-2016, 12:06 PM   #10
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Thanks for the information guys! I greatly appreciate the level of detail provided.

I do think that you're on the right track regarding it being a carb problem of some sort. As mentioned, it seems to run fairly strong if I feather the throttle to get it to a higher RPM. However, the top speed was still very limited when I tried it out before, but that was before resyncing the carbs according to the service manual.

One of the biggest reasons that it seems fuel related is from what I found this morning. I forced it to bog down around 15 mph this morning, then flipped the choke on all the way to give it even more fuel. It bogged down even further and dropped below clutch engagement. I did the same thing again and flipped the choke to the mid position to give it less fuel than at full choke, and it did the same, albeit at a slower rate. This definitely suggests to me that the bog is from being overly rich.

So, my plan is to pull the carbs, clean and inspect them thoroughly again, and check the needle clip position in case one of the previous owners put it to an extreme. I'm going to replace all of the fuel lines (some are in semi-questionable state) and get new (known size) pilot jets on order, and possibly order some smaller mains. The 370 is on the lower end of what I should be using according to the jetting table, but, as many people have noted on the forum, the stock jetting is often very rich.

Thanks again for all of the help. It's definitely nice for someone new to the application such as myself.
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