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Discussion Starter #1
Hey all, Today was the first day that we could get out and ride here in MN, so we got a good 10 inches of fluffy powder to play in.   A few of us took a ride, and after about 5 miles we were going through an open field, so I opened her up on some harder pack and caught up to the group.  While I was at full throttle (approximately 10 seconds) , she popped, and then acted like she fouled a plug.  We pulled the plug on the clutch side, and I noticed a nice hole in the piston right below the spark plug.

I tore it down, and the cylinder looks to be ok, so I know I need to get a new piston/rings, but is there anything else I need to look at so this doesn't happen again? The sled has about 550 miles on it, has been raced in the past, and to my knowledge, has never been apart before.   It had about a gallon of older fuel in the tank, but I topped off the rest of the tank with fresh gas.  It's jetted with 260's, which I think is ok...temp here was about 30 F, with elevation less than 700 ft.
Running NGK BR9ES spark plugs.

Any advise as to what may have caused the failure?  Does not appear to be a lot of carbon in the chamber, so I don't think it was that, but I really don't want this to happen much, as we only seem to get one good snow a year here.

Thanks for any help you can offer.
 

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I would be cleaning your carbs and look for any obstruction in the main jet? You also might want to check your crank seal for leaks. A hole in the piston is a lean condition.
 

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Discussion Starter #3
I would be cleaning your carbs and look for any obstruction in the main jet? You also might want to check your crank seal for leaks. A hole in the piston is a lean condition.[/b]
I can certainly check the carbs...they were just cleaned, but there's nothing to say it may have sucked something up to lean it out.

As far as the seals, wouldn't it run funny i.e. not idle, or idle real high if something was wrong with a seal?

Thanks for helping. :)
 

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i'd be checking the screen in the tank, inline filter,carb boots,and carbs. does that have a timing advance switch ?
 

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Discussion Starter #5
i'd be checking the screen in the tank, inline filter,carb boots,and carbs. does that have a timing advance switch ?[/b]
I can certainly check all those things as well. And yes, it does have a timing advance switch....which is set at 0 if I'm not mistaken.
 

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Discussion Starter #6
One more thing...The cylinder appears to be just fine, so can these be honed, or should I just leave it alone? I'm assuming they have the Nikasil bore, and I never know what you can or can't do to them. :confused:

Thanks.
 

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I would wonder about fuel condition too. Did your fuel have ethanol in it by chance? There is some reason for the overheat condition. It could be running lean or otherwise burning hot. There has to be a reason. Are the carb adapters tight and free from cracks? Is the whole carb ventilation system intact - tight carb boots and all baffles and foam in place? Check the filter to make sure it isn't plugged. Was the piston failure on the same side as the pump impulse tube? Is the impulse tube intact, not leaking air?
 

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Discussion Starter #8
In Minnesota, to my knowledge, all the gas has Ethanol in it...used to be only in winter, but I'm guessing that it is year round now.

I'll nose around and make sure that the carb boot isn't cracked, and replace the filter, just cause it's easy to do. I'm not sure which side the impulse tube is on...never bothered to check, but I can check that as well.... I'm inclined to think it is only a lean condition on the one side, cause the piston on the other side appears to be fine.

I wonder if somehow this piston got hot earlier in life, so it was thinner than the rest of the crown, and finally melted through when I was wide open? Being it was raced eariler in life, I'm guessing that the jetting was changed fairly often...maybe they went a little lean once. :unsure:
 

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Hole in piston means sudden lean condition. ie cracked or ripped carb boot, plugged main jet. which does'nt mean it is plugged now. if there is debris in the float bowl it can and will at some point float up to the main and plug it. when the engine dies it floats back to the bottom leaving no trace evidence. Piston smear on cylinder walls or crown of piston or exhuast side melted away is slow lean. ie crank seals, slightly wrong jetting. This is not every case. I'm not the know all guru. But you must investigate and find some reason for the failure or you will be doing the same dance again. Best of luck..
 

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Discussion Starter #11
Got the new piston in it, and did some checking around to see what size jets were stock in it. I bought it with 260's in it, and I guess the stock jets were 320's. So, I'm inclined to think that is why I burned it down. I've got 320's in it now, and although I just got it back together tonight, it seems like it runs stronger. We'll get it out on the snow tomorrow and really find out.


Thanks for all the help!
 
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