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Discussion Starter #1
I bought this sled a few weeks ago, it had 6500 miles on it owner said he was the original owner, no motor work has ever been done to it, claims it has been taken care of and not raced or beat to hell or anything. He started it up (4 pulls), ran and revved up fine but idled high (about 3k), did a compression check right in front of me, 118 and 120 psi. I took it out this last saturday in the first 6-8 inches of this weekend's monster snowstorm, busted trails for about 20-30 mins, then shut her down. Went to start it back up about 10 min later, and ended up having to choke it to get it to start. Took a few laps around the house, less than 5 min of riding, and it just seemed like it was losing power and then it backfired a few times. Drove her in the garage pulled plugs(light brown) and did a compression check to find 75 psi clutch side and 60 psi fan side. Took the heads off and the cyl walls look nice, very smooth, cant visibly see anything wrong with the top of the piston, head gaskets, or gasket mating surfaces. I pulled off the exhaust and looked thru the port and i think rings and piston sides look ok? Im not really sure what to look for there. I put it back together and now 75psi in both cylinders. By this time it had cooled down almost completely. I looked at the inside of the heads and upon close inspection noticed what looks like three partial hairline cracks starting to spread out of spark plug holes(about 1/2" to 3/4" long). The cracks are almost identical on both heads. It looks like the cracks are only skin deep tho, i mean like smaller than a hairline crack, and they dont look deep. I say crack for want of a word that describes something even smaller than a crack. I put a film of grease over the "cracks" on one head, put it back together and compression tested again with the same results. Also, the top of the head is completely dry with no evidence of fuel mixture leaking/blowing thru the top of the head or around the sides of the head. A couple ideas i have are blown gaskets, warped heads, cracked heads, worn/damaged rings/pistons, or glazed cyl walls? Im waiting for a repair manual to hopefully arrive by the weekend before i tear into it any more. Any ideas would be helpful and appreciated.
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Did you have the throtle wide open when you tested? I would pull the jugs and atleast do a set of rings (most likely pistons too)!
 

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It makes quite a difference. If you are pulling it over, make sure to keep doing it until the guage stops.
 

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It will.. Try it again. Sounds like its time for a teardown tho
 

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Discussion Starter #7
Had both plugs out when i tested, tried it again with the throttle wide open, no difference, still only 75psi. I lapped the heads flat(dont think they were warped cuz they cleaned up real quick) torqued them back down with new head gaskets and retested, still only 75psi. So i took the jugs off, found some light scrapes on the bottom half of the cylinders, and some burn marks on the sides of the pistons. nothing looked really bad, but i honed up both cylinders and plan on replacing rings/pistons. Also noticed my carb boots were cracked, so ill have to replace those too, kinda wonder if they leaked and caused the initial burn down.
 

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Discussion Starter #10
Yep, and there were also some light scratches on the piston sides too. Just gonna take the jugs to work tomorrow and measure them, the owner told me no motor work had been done, but id rather not risk getting the wrong size pistons/rings in case they were bored over in the past. Hopefully i can order parts tomorrow night and put it back together christmas weekend.
 
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