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The sled has 1500 miles on it, I had it out two weeks ago, put 28 miles on it no issues. Last weekend at about the 15 mile mark I lost acceleration and the motor quit. I could get it started again but could tell it was only running on one cylinder. Today I changed the plugs and dumped half a bottle of sea foam into the tank thinking maybe carb issues, it starts and will idle on still one cylinder. It pops and backfires when I open the throttle. Any ideas? Picture below is the old plug I pulled out, it was sticky. I very new to snowmobiled and have limited knowledge of two stroke carburetorated engines. Any trouble shooting ideas would be greatly appreciated. I have a trip planned next weekend and need to get this thing rolling. I have video of it running after I changed plugs, etc but do not know how to post if it would help diagnose.
 

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If you have the gauge, do a compression check on each cylinder. If you do not have a compression gauge, go buy one.

I suspect you have a damaged piston. You need to check with compression gauge to confirm, or get the wrenches out and remove the cylinder head to look at the top of the pistons.
 

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Hi.

Definitely check compression - as mentioned in above post. PSI should be above 115 (?) and within 10% of each other. If PSI looking good, then investigate electrical area next. Hopefully, both pistons are still good.

Basic electrical troubleshooting idea.

- Start / idle the engine and pull each spark plug cap off (while wearing dry snowmobile gloves). If engine dies, that specific side of engine is good. If it continues to run, that's the side of engine with bad electrical problem. - With known bad side of engine, swap plugs. Then swap Ignition coils. Does problem stay on bad side of engine or follow the moving parts?

Hope these simple "swap around parts" ideas help.
 

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As a proactive research task.....

Search this forum and Google for Stator troubleshooting details. If wondering, I had a previous Polaris and for no reason, its stator went. When the stator goes, its spark disappears. Only investigate Stator output if electrical problems remains on same side of engine (as you swap plugs & coil with attached wire and cap) around. After you find good stator testing info, cut / paste to keep it - in case you need this info in the future. Hope this helps as well...
 

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in addition, i would check the spark plug caps easy check or buy a couple, my kid races mx just about every race there is someone fighting a misfire and alot of the time its the plug cap. my second add is the fuel pump, I have had a couple pumps go bad lately due to the ethanal gas attacking the membrane.
 

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Discussion Starter #7
Gonna pull the carbs off this weekend blow out all the lines, test compression, new coil, new fuel pump. Fingers crossed
 

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Good approach.

When investigating root cause, do try to check the things that "appear" to work as well. Many, many years ago I had a sled engine that randomly would only ran on 1 cylinder. Found the side of engine and confirmed no spark (by pulling its cap off using dry gloves). Pulled its plug, checked its spark via grounding it and it was good. Re-installed the plug and no go! Very weird. To make a long story short, that specific plug would NOT fire `under compression`. Scientifically impossible but after changing that specific plug, the engine purred like a happy kitten. Lesson learned: Double / triple check parts on the working side of engine - even if they appear to manually test properly on the bad side of the engine.
 

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Blown cylinder
Yup. That will do it. Thanks for the pictures! You know what has to happen now wrt repairs. The question is, why? As the engine comes part look for evidence of what could have caused the melt down. eg: plugged jets in the carburetor, worn leaking crank seal(s), intake leak/damage to carb boot or bad reeds, loose carb clamp, ....

Do not just pop a new piston in and repaired cylinder on without knowing what the cause was.
 

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Discussion Starter #12
Face de Ace, you nailed it right away. I’m gonna end up taking it into the shop. I don’t have the tools or know how to tackle this. I’ll report back what the actual diagnosis is
 

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So it’s been awhile long story short I got out on the trails got about 10 miles on the new piston and the sled died again. Lost compression. Scratching my head a lot. Ended up taking the head off and saw damage to the same piston. Then it hit me, when I removed the first piston the arrow was pointing the wrong direction. The new piston was also installed With the arrow pointing the wrong direction. So I removed both pistons, both cylinders and had them bored .10 over. Bought two new pistons to match the bore and installed. After all of this was complete the sled started but I feel like there should be some carb adjustments to satisfy the larger carbs. I also noticed through troubleshooting the spark seems a little weak. Is that possible, is it either spark or no spark? Can you have weak spark? I replaced the voltage regulator/coil last season. Any way to test if I am getting adequate spark, and also does anyone know specs on carb adjustments for the over bore? Comments are greatly appreciated you guys are great!!
 

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I doubt that any carb adjustment will be required, Polaris jets their carbs rich from the factory. You could always go up a size (or half size) on the main jet and maybe one lower on the needle to raise the needle in the midrange to test. Check the plugs once in a while as you break it in. After you get it broken in to your satisfaction, do a "plug chop" at wide open for about 1/4 mile, then another at mid throttle. The first test will check the main jet, the second will check the needle setting.


A plug chop requires a fairly long straight stretch or meadow, when you get to the 1/4 mile mark, hit the kill switch without moving the throttle position. Pull the plugs and examine for color. You should have a cardboard tan color. White would indicate a lean condition, black is rich. You can also take a look at the piston wash each time. A small bend-a-light works great for that. There should be a thumbnail sized "washed" area in the carbon where the intake ports are in the cylinder. A bigger wash indicates rich, a smaller wash (or no wash) indicates too lean.



It's better to have those fanners a bit rich than too lean, so error on the side of too rich!


Good luck, think snow!
 
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