help diagnose this piston failure

G

goldenmm

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Jan 5, 2010
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7
Age
52
Location
Colorado
I need help with my MM700. Did this piston fail due to detonation caused by a lean condition? Or does is indicate overheating or a possible coolant leak since both the front and back are scored? Or is there a oiling issue? The first time this happened, the oil cable came loose from the throttle and the #1 cylinder and pistons smoked and it obviously failed due to lack of oil. The sled had around 5k miles then and I replaced the #1 cylinder and all three pistons. A few months later the #2 cylinder went and I suspected it was due to age and overlooked damage from the first failure. So I bought 2 more cylinders and dropped in more pistons, new gaskets, etc. The previous owner claimed he never had to do any engine work, and I did screw with the carbs when trying to install triples, but I am pretty sure I got them set back the original. I run yamaha 2s oil, but I think he ran bulk oil from the snowmobile club. Please help me get it right this time, what does the damage on here indicate?

Thanks,
JJ
 

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If it wasn't pushing coolant out the overflow then the head gasket wasn't leaking. If it wasn't puffing white smoke, then it wasn't burning coolant.

Did it overheat? Was this a "cold seizure"?

It doesn't look like detonation from the last pic, the dome looks fine on the edges and there is no damage to the ring land in the 3rd picture (intake side) so I'm betting lack of oil or insifficient.

Check the oil pump drive worm gear from the crank... might be stripped with those high miles and not driving the pump?
 
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Dry on oil/LEAN............. This early in the year I see this a LOT simple due to the fact of dirty carbs. 5000miles is no sweat for these triples and i dont even consider it "High Miles" I would also rule out the cold seizure as that would show a "4 corner seizure" Keep me posted....
 
Thanks for the input. Although a bit of a newbie, to me it seems like I hardly have to add that much oil to the reservoir, and it smelled pretty 'gasy' when we pulled the plugs in the field. Plus it refired back home and smelled more like gas then 2-stroke smoke. Unfortunately, the oil pump sounds like a major project. But as I said before, I want to get it right this time.
 
1st check your jetting! Once you rule that out and get her back together, Premix your gas @ 50:1 in the tank, fill your oil reservoir full, ride it.......monitor your oil consumption! You should only use about 1 qt of oil for a full tank of fuel burned... that will tell you if your oil pump is operationg properly with out the tear down
 
Johnny Quest, please explain how dirty carbs can create a dry oil / lean situation. And do you mean fuel lean or oil lean? I ran it about 50 miles once no problems this year. Then, 24 miles with half of that towing my buddy out. Then, about 3 with new plugs until seizure. I replaced the plugs because it was a little cranky about starting cold. The carb boots look fine.
 
The piston wash suggest just what Johnny said...lean situation. Just have to find why...check your crank seals for leaks.
 
Fuel is mixed prior to entering the carbs not in the carbs..... therefor if your carbs passages/jets are dirty your engine is lean on oil and gas, causing seizure! And also it takes usually 100 miles or less for this to occur
 
I don't see detonation(no melting of the piston exhaust side edge),i see over heat/ running hot and oiling issue. see lots of blow by too.

you said you had a oiling issue before did the piston look like this one?

if you said "it seems like I hardly have to add that much oil to the reservoir" that right there should tell you something.
 
Johnny / Luke;

Thanks for the advice. Now I have 4 good places to look, crankseals, oil pump, jetting and dirty carbs. Although I add Sta-bil, and syphon out all the gas I can before summer, do you still think the carbs could be dirty? Or is Sta-bil a whole new can of worms?

I will keep you posted as to what I find.
JJ
 
i would say this is from running lean on oil, from the look of the piston top no wash, was this a 3into one and you changed it to three singles(triple pipe) if so you must rejet what do your plugs look like? being that the thrust faces of the piston are the ones that took the brunt of the punishment, started on exhaust and moved to intake this would tell me lack of oil now piston top was getting cooled by fuel or scoring would have started above the top ring just my 2 cents
 
Daman/ChevyBoy/MiViper,

Yes, the oil cable is actuating the pump. Sorry I could not get a good photo of the plugs, but to me it looks like the ones that had been in there for some time were brownish black suggesting plenty of oil, with a little wear on the electrode. The new ones with less than 3 miles before seizure were a little white on the grounding conductor, but really no other color.

I plan to check the crank seals, then the carbs. Just need to get my hands on the proper tools.

thanks for the input. JJ
 
chevyboy,

I gave up on the pipes and went back to stock exhaust and carb settings / jets. Everything worked fine last year.
 
follow-up piston failure

So I pulled the crank and had a machinist friend take look. 5.7K miles (2000 MM700) with alot of towing skiiers toward the end of its life. Like I said before, 2 previous seizures on this motor. Well, end play on rods is 1.5mm (1.5x factory specs). Also, most of the crank bearings felt pretty loose except for the 2 next to the oil pump. Upon further inspection, the #2 cylinder did not look so good either. Alot of blow by on mag side and center pistons. Carbs looked fine, still not sure if the end seals were the problem, but leaning that way. Based on the bottom end parts cost to rebuild I am probably going to part it out. Although I like the sled, I have to make a tough decision here and parting it out seems to be the way to go. The clutch, carbs, and everything else is good. Thanks for the help. ;)!
 
Could always find a used motor for it too...thats what I did in my 97 700 Sx this year. I got it in a straight swap with some extra parts I had laying around, but he was selling it outright for around 600. Just an option, but if you have some good parts on there could be worth it to part it and upgrade to a newer sled.
 
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