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Engine failure

B
Jan 23, 2008
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5
8
The piston wash doesn’t look like it was lean... I would check to insure something didn’t get ingested into your in-take. To me, it looks like something was banging around and thus you have indentions in both your head and piston. If it was a lean burn, you would have scaring on your clyinders.
 

kidwoo

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Dec 28, 2008
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Any ring bits get in there?

Almost looks like some rod bearing indentations too.


Pull the jug and see what's up with the rods. If that didn't have something metal break loose, that's some seriously nasty det.
 

snoluver

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Dec 11, 2007
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Definitely a foreign object got into the cylinder. Probably a broken ring. I wouldn't think a skirt broke with only 980 miles. But a ring could have been stuck from the sled sitting a long time. That would kinda make sense with that few miles on it.
 
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CATSLEDMAN1

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Nov 27, 2007
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by the looks

most of the time when I've fixed arctic cat motors with this look its the result of DETONATION and its been going on for some time, maybe 50-100 miles, maybe since new.

the most likely cause is lean fuel air mixture causing the toooo lean fuel mixture along the outer edge of the piston to explode with a bang instead of the ideal soft push ( technical term as pointed out detonation/deto ), most likely an air leak. Seals on these engines don't go bad sitting for that length of time. Air leak from base gasket very likely, loose stud bolts/nuts. Leaks around the reed valves/reed cages, blown a few belts, crank seals with belt threads stuck in them.

Just a little detonation now and then for 900 miles will anneal the piston crown which allows the ring lands to widen and the rings to flutter and not transfer heat off the piston crown, then under a load in the rpm range where the deto is the worst , a ring end may weld itself to the piston wall for a mili second, break off and go around through the transfer ports and dingle up the outside edge of the piston.

However I would expect when you pull the piston, you will find rings intact, skirt intact. When the center of the head shows little or no scaring, the outside edge of the piston is eroded from little bits of aluminum becoming liquid for a moment and bounding around . The pop you heard was the last straw, not the start of this problem.

Timing could cause this, but the timing on these motors is same for both pistons, one good piston one bad piston, rule out timing.
 

kiliki

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Dec 27, 2008
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if it sat for a long time I hope you drained the old gas out of it!
If not you can get det and this can break off the ground strap on the plug and this will leave the little J marks on the piston top.
did all the plugs have the ground strap?
did you drain the old gas?
where any of the plugs loose?
 
D
Feb 10, 2016
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39
So I pulled the jug off and the rings and skirt still look intact. It was definitely a foreign object. Almost thinking it might be a bearing that let go. Found small peices that look like the roller bearings. I need to check for play in rods or pull the motor and see if it was a crank bearing or main. Does that sound right. I hat to pull it. But it was suggested I do it regardless to get any metal out. Any recommendations?
 
J

jim

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Nov 26, 2007
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Boise
I think it's a rod bearing based on the size and shape of the dents. Pull the jugs and piston and you will know. You can also look inside your exhaust pipe...likely more metal inside there to tell you what happened. As for cause, the piston to me didn't look detonated because the edge on the exhaust port side is still square (where I can still see an edge, that is). I have detonated pistons before and the first part of the piston that detonates is the exhaust side...and you will see rounded, sand-blasted looking piston edges. I think a chunk of metal let go. Maybe the top end rod bearing went via corrosion? With low miles, wonder if it sat somewhere where it got corrosion and let go. Have seen this before with outside stored motors that sit around...the hot/cold temp outside condenses and corrodes.
 

snoluver

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Dec 11, 2007
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I think it's a rod bearing based on the size and shape of the dents. Pull the jugs and piston and you will know. You can also look inside your exhaust pipe...likely more metal inside there to tell you what happened. As for cause, the piston to me didn't look detonated because the edge on the exhaust port side is still square (where I can still see an edge, that is). I have detonated pistons before and the first part of the piston that detonates is the exhaust side...and you will see rounded, sand-blasted looking piston edges. I think a chunk of metal let go. Maybe the top end rod bearing went via corrosion? With low miles, wonder if it sat somewhere where it got corrosion and let go. Have seen this before with outside stored motors that sit around...the hot/cold temp outside condenses and corrodes.

I agree with this 100%. Most likely a bearing failure from sitting. Motor needs to come out and case split to ensure all debre is removed. You will find the failure along the way...
 
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