[attachmentid=19753]Could use some advice. I was running across lake -20 C or so when sled died. I did compression test and found fan side cylinder was toast. I pulled off head and plug and saw plug was melted and top of piston had a dime sized crater and edges disintegrating next to rings.
Plug was running extremely hot obviously but I'm not sure what causes this. Can anyone help?

Thanks Rob

looks like some detonation. how many miles are on the sled?? i had a 380 that ended up with a piston like that.

First place I'd be looking would be inside that carb at the main jet, see if it's correct for the conditions or more likely partially plugged causing a lean condition.

check gaskets for leaks as well. also check carb boot to see if it was sucking in more air than it should be. carb boot might be cracked from being old.

Originally posted by robert007@Mar 13 2005, 08:08 AM
[attachmentid=19753]Could use some advice.* I was running across lake -20 C or so when sled died.* I did compression test and found fan side cylinder was toast.* I pulled off head and plug and saw plug was melted and top of piston had a dime sized crater and edges disintegrating next to rings.
Plug was running extremely hot obviously but I'm not sure what causes this.* Can anyone help?

Thanks Rob
410636


Sled only has 1700 km on it. Main jets in carbs are mj140 and circlips are at #3 postion.
When I took the jugs off it I was able to remove all the mounting rods by hand. Not sure if that meant heads were loose and air was getting in. I was thinking of using locktite on studs to reinstall them..what do you think? Also i did clean out carb after blow up and mainjet was gummed up inside but slide needle was still penatrating ok???

Could you please explain detonation?

You leaned her out. Msot likely at -20 it developed a bit of ice under your main jet on that side and cooked it. ALWAYS RUN ISOPROPYL ALCOHOL IN YOUR FUEL!!!!!!!!!!!!!!!