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Discussion Starter · #1 ·
A little information to get started.

I had the stator go blow during the 1st cross country race of the season. I installed the new stator and went trail riding and the sled ran great. I packed up and headed to the 1st sno-x race of the season. I fould a plug while warming the sled up on the stand. Fresh plugs and off I went. It ran good during the first two heads. I fould a plug on the starting line of the main and pulled off. I just brushed if off as one of those moments. I went to the the 2nd race of the season and fouled a plug during every heat! I loaded up and went home to the shop. Topped the tank off on the way and picked up some fresh plugs. Cleaned the carbs and inspected, didnt see anything that looked odd. Went trail riding and it ran great. Packed up for the 3rd race and fouled a plug loading it on the trailer.

Not sure what the deal is, but it just started to happen out of the blue.


2002 mxzx 440 LC with 03 zx 440 carbs. 310 PTO 300 Mag 25 pilots scew 1 turn from seated.

New needles and seats, also rebult engine 2 seasons ago with about 8 heat races on it. 155 psi each cylinder.

Any ideas on this issue?
 

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Maybe you have a weak coil, find out what its/they are supposed to put out and test it. Just an idea
 

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same plug fouling everytime? and always at just off idle?

Do the Doos fire both cylinders at the same time?
If so, try swapping plug wires, see if the problem moves.
May be as simple as a bad plug boot.
 

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Discussion Starter · #4 ·
Both fire are the same time and switching didn't seem to help. Im thinking carb sync?. I swapped carbs and it seemed to stay at the mag cylinder. Seems to only do it with low fuel level in the tank. HMm im still playing around with it.
 

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440racer said:
Both fire are the same time and switching didn't seem to help. Im thinking carb sync?. I swapped carbs and it seemed to stay at the mag cylinder. Seems to only do it with low fuel level in the tank. HMm im still playing around with it.
maybe too much oil at idle?? clogged oil routing line or is it a premix? bc if it was, maybe your getting settling issues? what is the psi in both cyls? sorry for the rambling
 

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Discussion Starter · #6 ·
psi is around 155 each cylinder and I premix 3 oz to every gallon.
 

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wildcat-700 said:
are you sure you dont have a crank seal gone on the mag side i had this happen with a 500
If he had a crank seal gone bad that cylinder would be burning lean. Not going to foul plugs. The way I see it he either has a weak spark, or its a carburation problem. Checke needle and seats, not just look at them but pressurize them and make sure they are holding pressure. Check pilot and main, make sure needle jet is in the right position.
From what ive read I would look after spark pretty hard, the system has changed. You could have made an error, or have a fualty part. Second guess all your work always. Make sure plug wires look good and are not cracked and boots are on securely, and not arcing. Could try a hotter plug but that doesnt fix the actual problem.
 

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Discussion Starter · #9 ·
Update:

I pulled both carbs again and set both slides tell they lift at the same time exactly, then turned my pilots out 1.5 turns i had them at 2. adjusted my idle a bit and filled the tank with 5 gallons of fresh fuel.. Sled hasn't fouled a plug after idling/running on a stand for 30 minutes. So it was either the carb sycn or the fuel being low? I guess I will find out this winter once the tank runs low again.

Thanks for all the advice.
 

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Discussion Starter · #10 ·
I had some free time so i figured I would run the sled again. I shot some primer into it and fired it up. let it idle 2-3 min hit the throttle a bit and she fouled the mag plug again. so I changed them out fresh and fired it up again. I didnt let it idle long and then hammered on it and she cleaned up. pulled the plugs and they looked great ran like this for 20 min on the stand. I also swapped plug wires when running the new plugs. not sure what to think at this point.
 

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Discussion Starter · #12 ·
temps dropped down nice and low the past few days. I figured I would moved the sled out of the shop onto the trailer so the garage had some room.. I had swapped the plug wires when I last played with this issue.. What do you know after warming the sled up and hitting the gas I fouled a plug. It was the PTO plug this time. Also blew out the low beams! So im thinking coil since it switch cylinders after swithing the wires.
 

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Discussion Starter · #13 ·
new 80.00 coil and the problem is still present! I started it and pulled the plug caps off just a bit to check spark.. One wire still sparks between and stays runing, the other will not. I did notice at this point when removing the wire that it would arc between the coil and the jack shaft if I reinstalled the wire back on the plug it wouldn't arc. Still having this issue with one side!

Any advice!
 

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Discussion Starter · #15 ·
trigger coils?
 

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The trigger coils bolt to the crank case. Theres two of them and they need to be gapped of the mag. If you have one to far out maybe its not triggering all the time. Im grasping at this point.
 

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Discussion Starter · #18 ·
I ordered new plug wires just to be safe! A lil to much $$$ for plug wires 80.00 for the set. So this will make me feel safe that I am working with a good coil and wires. I pulled the cover and started looking for the trigger coil.. I believe I see it, back side of the engine with two bolts holding it on. Magnetic and it also has glue on back side to help secure it to the crank case. Now just to find the spec for the cap for a 02 440 race sled.

Another odd fact. Seems when I did the rebuild I ended up with a #8 base gaseket? Im pushing 155-160 psi after a hard breakin during sno-x season. From this im thinking the low comp heads where not in the sled like stated at time of sale.
 

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Im no race expert but I would say a #5 base gasket would be right. And yes you have found a trigger coil, im assuming they are fine now. You never touched them and I dont see them moving on there own but check it anyway, cover all your bases. That is a lot of compression, what octane fuel are you running? Anyways thats deff not a low compression head.
Heres what I would do at this point, go get an inline spark tester that lights up when charged(if you dont have one already). Put it on, put the machine on the stand and watch that spark tester very very closly. If you have a plug foul out you should see if you had spark falling off or completely dissapear. You should also be able to tell if you have weak spark on that cylinder. What plugs are you running?
 

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Discussion Starter · #20 ·
I run Br9eya or br9es for plugs. new plugs wires are still on order, I guess the dealer is having a hard time find them. I run 91 octane for fuel. I only moved the trigger coil when I did the rebuild a few yrs back. When I reinsalled it I just put it back like it was, the black glue left on the back side helped..
 
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