She sled is a 99 summit x 670 with a rebuilt engine and about 400 miles on it. bought the sled from someguy who rebuilt it himself and rode it once. bought it 3 moths ago and put on several hundred miles. our problem has been in the past two days and it the first time its happened. i was climbing a hill with wide open throttle, and one of the cylinders went dead, so i took the plugs out and the plug in the right cylinder had no gap in it, and there was a couple of scrapes on the plug and two very small dents on the top of the piston just below the plug. the plugs are NGK BR9ES, which is what every sled ive ever ridn takes. wondering why the plug gap was smashed together and why the piston is hitting the plug... the left piston is fine. I haven't the slightest idea why and i would apreciate any help! Thanks-Eazy

you should check for a bad bearing either on the connecting rod or the pin in the piston.Is it making any knocking sounds while running?

I had this happen to me 1998 670 summit, rebuilt the sled was going across a lake and dropped a cylinder. Checked the plug and it was closed, first thing I thought was bearing,(crank or connecting rod) put in a new plug and drove it to the truck. Got to the truck and same thing running on one. Tore the engine down and found out a chenk of ring was missing. When I rebuilt the engine the first time I didn't do the ring end gap,I just stuffed it together maybe thats what happened on your rebuild????? on my secound rebuild we filed down the rings for the proper fit and 300 miles later it's still running strong.This may not be your case but it's a place to start,good luck

No knocking sounds. the gap dosn't close instantly either, today my girlfriend drove it and we never went faster than 20 for about 7-8 miles then went home. shut it off for about 10 minutes and went to start it up again and i wouldn't. took the plugs out and the one was about half the gap that it was origanally. weve been having problems with the pull start, keeps braking, a snowmobile dealer said there was some kind of recall on the whole pull start system, and the half an aftermarket pull start that they recomend for about $150. whats my next step? shop? open up the engine? wonder how much it will cost to have it checked... let me know what you guys think. Thanks alot,-Eazy

sounds like you lost a rod bearing

Quit driving it.
Something is wrong inside and if you keep driving it, you will have more things broken inside.
Yes, you will have to open it up to find out what's wrong.
Good luck

Jeff