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Discussion Starter · #1 ·
Hi, I have problem with ZL 500 carb 1998. I have rebuild top-end and clutches and some orher things and it is working great, but..

It starts with first pull when warm and 2-3 pulls with choke when cold at -20C.

When started cold, it idles at steady rpm as long as I want. But after ride in forest/deep snow it idles very low and after WOT at lake and stopping to idle rpm very fast drop and engine shut down. Sometimes it will start up with first pull after ride and either idles normal or low. Sometimes it takes 2-3 pulls and idles low. Then after short hard driving it may idle normal again.

It sound and smell like it is very rich, piston head seems to be wet or at least there are no carbon build-up and color is tan or light brown and look wet or shiny. Plugs are sometimes wet and bit black and sometimes they seem to be little lean (not white, but gray, aint that leanish?).

All above is very confusing, so I measures stator as I have already cleaned carbs many times, changed flanges, synchronized and changed jetting. Main jet is 360 and needle clip middle. Have not replaced needles nor pilot jets. I have ordered them so will be replacing them maybe next week.

Stator cold:
49 ohm
420 ohm
460-470 ohm

Stator hot after idling as long as top water hose was warm and then revved up a little:
49 ohm
490 ohm
530-540 ohm

And finally, I discovered that when I shake sled side to side idle will drop from 1600 to nearly 1000 rpm and slowly rise back up. Could that be fuel flooding or normal? That would explain slow idle after riding.. Maybe.
 

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Discussion Starter · #2 · (Edited)
What I have replaced/done (I may have forgot some things):

  • pistons/rings/pins/gaskets/one cyl replaced
  • throttle/oil pump cable replaced, throttle synced and adjusted
  • choke plungers and springs replaced, chokes adjusted
  • ignition coil replaced
  • plug hats replaced
  • plugs several times (BR9EYA) replaced
  • fuel pump, hoses to carbs and impulse line replaced (impulse line hose clear and no fuel in it when running)
  • fuel filter in-tank and before fuel pump replaced
  • carb flanges replaced
  • cleaned carbs several times
  • originally main jet was 340, tested with 350 and currently 360 installed
  • needle clip in middle position
  • air/fuel screw 1 1/2 turns out from seated
  • idling 1600-1800
  • refurbished drive and driven clutches, aligned and adjusted
  • replaced belt
  • track aligned and tension adjusted
  • skis aligned
  • fuel premixed 2% in-tank (second tank going and after that no premix)
  • oil pump adjusted so it will engage when throttle is couple of millimeters pushed
  • exhaust pipe and muffler cleaned
 

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2005 Z570, 2006 Panther 570
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The only effect, that I can conclude, the rocking motion would have is on the floats at the bottom of your carburetors. The bowl fills with fuel and acts like a cup which the carburetors will drink from via the pilot jet (when at idol/throttle closed) which acts as the straw in my analogy. The float will close or open the needle valve which draws fuel into the bowl from the fuel line.
(This is my understanding of how carburetors work. Please correct me if I'm wrong.)

Have you checked the float arm? Is it possible the float arm is not adjusted and there is too much fuel entering the carburetor's bowl? Have you cleaned and inspected the needle valves on your carburetors? The tip of the needle valve is a cone shape. The older style needle valves were solid brass and sometimes the cone shape would wear over time due to machine vibration and wouldn't allow a full seal when the float arm pushed the needle valve up. Newer needle valves have a rubber cone tip. However, this doesn't mean they are immune to being compromised. I'm assuming your machine is new-enough to have a rubber-tipped needle valve

A new needle valve for your Mikuni VM38 will run you around $40. Arctic Cat part no. 6505-171

HERE is a parts diagram for your carburetor.
 

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Discussion Starter · #5 · (Edited)
Thanks. That is what I was thinking about also.

Actually I only removed the needle valve clip that holds valve inside the valve body, to lower needle down so I could spray through the valves.

Float arms I adjusted so that they were at same level with gasket surface when carb was turned upside down. I then blow air by my mouth through fuel inlet line to confirm no air can get past the needle valve (when upside down).

I have ordered rebuilt kit for both carbs, so will be replacing needle valves as soon I receive them.
 

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Discussion Starter · #9 ·
Just an update.

I received carb repair kit a while ago and replaced everything that came with the kit. Also added 350 main jet that came with the kit and noticed that those I purchased earlier were wrong sized, even there were stamps stating they should have been 350. The hole was much more smaller than in the ones that came with the kit.

Now idle is steady in every situation and sled runs perfect.

I now can easily adjuat carbs just by moving the clip on the needle:

Around -15C...-5C clip in the middle slot.
-5C...-0C clip second highest slot.
+0C clip highest slot.

Now snow is smelting away so I think this is end of the season. Time to clean fuel tank and drain every bit of fuel out of the system and wait for next season.
 

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The needle clip is not for temperature compensation adjustment, that is done with the main jet. The needle jet position is for fine tuning part throttle operation. If start with a proper baseline carb set up you will be able to calculate what main jet you need for the temps of the day.
 

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Discussion Starter · #11 ·
That 350 is for about the coldest weather I will be riding and as far as I have understood I can adjust carb for warmer weathers just by moving the clip as main jet still is the boss there. Somewhere I read that moving clip one notch is about the same as replacing main jet about 10 units to same direction (richer/leaner) and keeping clip in the same position. I have been checking plugs when doing that and all seems to be OK and it does make sled ride better. It was way too rich at +5C with clip at second highest at middle throttle, when I moved it to highest position it was spot on with middle throttle and maybe a bit rich at WOT, because of the main jet.

Just to mention, I normally ride in the woods so not so much with full throttle. But sometimes there is fields and ice crossing so I need to have big enough main jet for WOT sotuations. And adjusting just the clip seems to be enough for that.
 

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Discussion Starter · #12 ·
Once again some snow to drive on.

Replaced (again) carb boots as earlier replaced ones were ruptured already after one year.. Got better quality ones from Dennis Kirk and material+design seemed alot better. Also cleaned carbs and re-adjusted/synced. Main jet is 350 on both, installed new choke cable, new stock needles and something else smaller stuff. Needle is set to second highest groove, even at -15C degrees it is too rich when at middle position. Midrange and WOT are now spot on.

Idle is still a problem, it seems that plugs get fouled/wet when idling. I got it to idle around 1600 rpm +-100, and after minute or two idle start to drop and it drops until engine just keeps running.
Pilot jet is stock 40, air screw out from seated 2 1/2 turn seems to work best but still not good. I dont wanna go further out with that.

I have ordered set of pilot jets, should I go straight to #30 or try #35 first to get idle problem sorted?
Or am I lost and it is not about pilot jet?
 

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Discussion Starter · #14 ·
Tried that earlier and it dies or gets very worse, depending how long choke is on.

Forgot to mention, it is not very responsive when taking off from idle or trying to go "walking speed", often I need to pump up the throttle to get it going. But as said, middle range and WOT are perfect.

So I believe pilot jet needs to be smaller to reduce fuel under 1/4 throttle, but how much would be enough, so even when it hits -25C degrees it is still on the safe side and not too lean? Should I aim air screw to be out 1 or 1 1/2 turns from seated position at -10C degrees, for example?
 

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FYI on pilot sizes : Pilots 40 and smaller come in 2.5 increments. Larger than 40 come in 5 increments.

often I need to pump up the throttle to get it going.
This is usually an indicator of a cutout problem moreso than a pilot. Cutouts increment by 0.25 but note that larger numbers are leaner with 1.0 being the richest. Changing a cutout will cause the pilot to act in the same direction as the change in the cutout ie: Going from a 2.0 to 2.5 cutout will make the 40 pilot act more like a 35 or 32.5 pilot.
 

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Discussion Starter · #17 · (Edited)
FYI on pilot sizes : Pilots 40 and smaller come in 2.5 increments. Larger than 40 come in 5 increments.



This is usually an indicator of a cutout problem moreso than a pilot. Cutouts increment by 0.25 but note that larger numbers are leaner with 1.0 being the richest. Changing a cutout will cause the pilot to act in the same direction as the change in the cutout ie: Going from a 2.0 to 2.5 cutout will make the 40 pilot act more like a 35 or 32.5 pilot.
New information, thanks!

What is the default cutway size on VM38 on ZL500?
That would be easier to test with larger cutway, but may be more expensive.. Also I have had thoughs to change slides as they seem to have some fine scratches on theis sides indicating some worn out, but nothing major. But that was just a though, I'll proceed with smaller pilot jet and see if that helps with the problem.
 

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Discussion Starter · #19 ·
I got 32,5 pilot jet, trying to install it this weekend. Shame we have +2C degrees outside at the moment so need to wait for end of next week to get some cold weather and to test how it works..
 
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