Michigan Brad
New member
This is my uncles sled and he purchased this sled new. 2 years ago we replaced the intake boots because they were cracked and it caused piston failure in the right side piston. But no cylinder damage occurred. It recently had another piston failure and it was taken into a shop and a leak down test performed and they said the crank seals were leaking and the reeds were bad. So just before xmas we rebuilt the motor and had the crank inspected and installed new seals and reeds. The shop had also replaced the fuel filter at this time. After the rebuild it's been ridden about 4 or 500 miles and just recently has not been able to go above 60mph just seemed to have no guts on the top end. So we changed plugs and still no change. Then we proceeded to clean and inspect the carbs. We used carb clean and blew through all the passages with compressed air but found nothing wrong. So a compression test was performed and the left (pto) cylinder had 110psi and the right had 120psi. So we pulled the cylinders and this is what the piston looked like. The corner that is melted is the exhaust side of the cylinder. I'm running out of ideas on where else this problem could be. 
Bad gas? PTO seal leaking, even though replaced?
Did the shop do the work or did you? Longshot here, but were the head and/or base gaskets replaced, with thinner versions than stock thus increasing compression?
How does the other side look, and the plugs? Pics?
Is the sled using a lot of oil, thus the extra oil is displacing gas in the fuel mixture?
Exhaust leaks?
Are you absolutely positive the carb and airbox clamps were tight and the boots were properly sealed at the carb and airbox? Also same for the Air chamber/Balance chamber sealed up to the carb boots, even though it has a little spring clamp? I had one burn down way back when, under warranty, and the dealer said the leakdown test showed an air leak at this connection. When reassembled they cleaned the parting line flash and used silicone sealer to be sure of a good seal.
Could doublecheck carbs for blockage and specs/adjustments?
Hopefully your cylinder is still good with that light of damage. If everything checks out ok, could pop another piston and ring set in there and get 'er running. Then check the PTO seal, case halves, base gasket, carb boots with carb/brake cleaner and see if there are any leaks that cause the engine to change rpm. Then run another leakdown to verify? Also keep an eye on plug readings and the piston dome for lean condition, even if every time the sled is run. Can do throttle chops at different speeds to check plug and piston for lean/rich. A lean spot may only be in one area of the rpm range/carburetor settings. Might try rejetting and/or raising the needles so it's rich and then backing down from there.
Did the shop do the work or did you? Longshot here, but were the head and/or base gaskets replaced, with thinner versions than stock thus increasing compression?
How does the other side look, and the plugs? Pics?
Is the sled using a lot of oil, thus the extra oil is displacing gas in the fuel mixture?
Exhaust leaks?
Are you absolutely positive the carb and airbox clamps were tight and the boots were properly sealed at the carb and airbox? Also same for the Air chamber/Balance chamber sealed up to the carb boots, even though it has a little spring clamp? I had one burn down way back when, under warranty, and the dealer said the leakdown test showed an air leak at this connection. When reassembled they cleaned the parting line flash and used silicone sealer to be sure of a good seal.
Could doublecheck carbs for blockage and specs/adjustments?
Hopefully your cylinder is still good with that light of damage. If everything checks out ok, could pop another piston and ring set in there and get 'er running. Then check the PTO seal, case halves, base gasket, carb boots with carb/brake cleaner and see if there are any leaks that cause the engine to change rpm. Then run another leakdown to verify? Also keep an eye on plug readings and the piston dome for lean condition, even if every time the sled is run. Can do throttle chops at different speeds to check plug and piston for lean/rich. A lean spot may only be in one area of the rpm range/carburetor settings. Might try rejetting and/or raising the needles so it's rich and then backing down from there.
Michigan Brad
New member
I did the rebuild myself. There was no damage to the cylinder and the other piston looks fine. The plugs that were in it were ridden on with the bad piston. I'd have to see if I can find the previous plugs. All the gaskets were ordered from the dealer and were oem replacement gaskets both base gaskets and the head gasket were new and not reused. The service manual says it should make 120psi which it does on the right cylinder. Also replaced the exhaust to cylinder gasket and used hi temp RTV on the surfaces cause they usually develop leaks. I checked the adjustment on the oil pump and its within spec and the machine doesn't seem to be using anymore oil than usual. It's 100% stock not even studded still has original track and all. It has about 17,000 miles on it and all the carb jets are the same size as it was equipped from yamaha. I will see if I can find the set of plugs we changed out earlier when we cleaned the carbs and post pics if I can find them. I will also check those connections at the boost bottle. I don't have a leak down tester. Is there somewhere I can purchase one or maybe even make one?