M7 melt down analysis

[sra1218]

*****updated see post on second page*****

2006 arctic cat m7

153 track

3750 miles

riding area: crested butte/irwin

average temp: 0-15 degrees

snow depth: 24-36"

elevation: 9000-10500

Snowmobile ran excellent all weekend until incident

7900- peak rpm on road 7700- in deep snow

detonation description: pto side, near y pipe, nickle sized depression not very deep

put 60 miles on sled this weekend when the pull rope broke. started sled with rope around primary and let the sled warm for ten minutes. rode the sled close to 15 miles towards the trail head when the check engine light came on and the sled jerked to a stop. average speed 30-60 miles per hour.

99% stock( all mods are not motor oriented)

only change made before the weekend was a stiffer primary spring (green and yellow slp). engagement went from 3700 to about 4800 in an attempt to chirp broadies.

now the interesting part,

my friend with a full mod m7 ( big bore 800) also melted down, but on the mag side. also a light melt down. similar circumstances. same gas


ideas:

bad gas?

crank seal?

??????????????

 

[skidooboy]

carbbed??? dirty pilot jets, gummed needles, and or gummed mains ... mid range lean burn down.

efi??? plugged injectors, bad gas, low octane, poor mapping... mid range burn down

first rides of the year claim more sleds from poor storage and old gas, than any other reasons combined.

Ski

 

[sra1218]

Sorry. Sled is Efi. This was not my first weekend riding. I've ridden the sled probably 100 miles before this weekend. The gas was fresh from the station 91 ethanol free. The sled had been running flawlessly up to this point.

 

[Phizzer]

Looks like the locating pin for the piston ring came loose...I bet if you pull the jug off, the locating pin is loose or gone.

 

[vinpie1]

Phizzer- I don't think it was the locating pin for the rings. In my experience if it falls out the result is a broken ring and scarring from the destruction. I am leaning towards a crank seal that leaks when hot. We both experienced major heat saturation due to the deep snow conditions. I have twin pipes on mine and usually have to stop now and then to let everything cool down under the hood. SRA1218's machine is completely stock. Check the wash on the pistons, there is no way he was running lean with a wash like that. My best analysis is that his pto side crank seal is letting air in when hot. I believe my machine has a bad mag side seal. I checked my seal with starting fluid the other morning when it was cold, and no increase in rpm, but I feel that it might only leak when hot (esp. saturated). Thanks for all the input guys. Keep it coming!

 

[vinpie1]

I'm running a D&D cast cylinder 800 big bore on my 05 m7. Its got two base gaskets and no timing key installed. I don't feel that I have too much timing in the mid range since there is no key installed. Feel free to chime in if you think I'm wrong. Thanks

 

[Phizzer]

Yeah you probably won't know until you tear it down. I was just looking at my motor when you posted that and the upper locating pin for the ring sits right in between those ports. My thoughts are with 3750 miles on a 2 stroke, it was probably due for new pistons. I've seen 2 stroke outboards (Yamaha's, Mercury's) with many hours on them loose the locating pin and the results are similar. I've even had older pistons from torn about motors that you could shake the pin out of the piston! But like I said, you won't know until he tears it down. AND the fact that both of you lost a cylinder on the same day is strange...

 

[Arctic Thunder]

Lots of wash, surely wasn't lean.

I had an M7 with a few miles bust the skirt off. Not very common but I suppose a person should think about pistons every 2000 or so miles on these.

One other thing, you said you had a broken pull rope, then fired up the sled and rode. This can be ugly if it catchs just right in the recoil. I did this one ride last year, when I pulled the recoil the knot and end of the rope were about to catch and raise hell with my electrical.

Thunder

 

[sledtahoe]

nycisil failure

The m7 are know for the nycisil to bubble or flake off which would cause seizure. Not that it happens a lot but it does and sounds like it could have been bad after that many miles.

 

[spiceyunicorn]

37** miles on stock/original motor things go wrong they dont run for ever

 

[Vern]

as perfect of groove as that is I'd also be inclined to think it was something along the lines of the ring pin coming loose. so 3700 miles is to many ehh? guess I'm gonna put this to the test this season, i got almost 4200 on my original internals and no plans on tearing into her this season. guess we'll see what happens.

 

[Arctic Thunder]

as perfect of groove as that is I'd also be inclined to think it was something along the lines of the ring pin coming loose. so 3700 miles is to many ehh? guess I'm gonna put this to the test this season, i got almost 4200 on my original internals and no plans on tearing into her this season. guess we'll see what happens.

I did the same thing on my 03.5 900. Ran that thing without pulling the head or anything, it ran nearly 5000 miles. But man when it went it was destroyed. Bent rods, etc. The piston finally just blew apart under load and it got ugly. Picked up a new motor dropped it in and away it went.

Thunder

 

[Thunder101]

I see the exhaust edge of piston eroded and a crater starting in the center, bad gas is my guess. I cannot explain the wash but aluminum has been getting burn up so,,,,

 

[sra1218]

I will be tearing into it this weekend. I will update the post with what i find. I went ahead and ordered crank seals with the piston kit, and plan on doing a bottom up rebuild.

Does anyone know of a good way to test gas for moisture content without attempting to boil it? It would be comforting to know if it was in fact the gas.

Steven

 

[Arctic Thunder]

I will be tearing into it this weekend. I will update the post with what i find. I went ahead and ordered crank seals with the piston kit, and plan on doing a bottom up rebuild.

Does anyone know of a good way to test gas for moisture content without attempting to boil it? It would be comforting to know if it was in fact the gas.

Steven

I can remember in the past draining the fuel from the fuel rail into a glass cup and letting it sit and see what you get.

Thunder

 

[sledtahoe]

3700

37** miles on stock/original motor things go wrong they dont run for ever

That many miles is really low for this motor in stock form is proven almost bullet proof..

 

[mikew5945]

Sorry. Sled is Efi. This was not my first weekend riding. I've ridden the sled probably 100 miles before this weekend. The gas was fresh from the station 91 ethanol free. The sled had been running flawlessly up to this point.

I agree with the 37** miles not being the issue. Yes, you should do a top end around there but that is not the problem. When I put a Boondocker on my M7 one piston was richer then the other. I believe it was the PTO side. I'll bet you got crap fuel and/or mixed with ethanol. It is the only common thing with your buddy. It is a long shot that the seals went at the same time in two different engines.
You had a well pronounced wash until the last ride where you ate the center of the piston. Everything being the same as the last ride except the fuel. That is where my bet is.

 

[sra1218]

I agree with the 37** miles not being the issue. Yes, you should do a top end around there but that is not the problem. When I put a Boondocker on my M7 one piston was richer then the other. I believe it was the PTO side. I'll bet you got crap fuel and/or mixed with ethanol. It is the only common thing with your buddy. It is a long shot that the seals went at the same time in two different engines.
You had a well pronounced wash until the last ride where you ate the center of the piston. Everything being the same as the last ride except the fuel. That is where my bet is.

vinpie pulled his pistons and believes he may have leaned his machine out. this would make it plausible that my crank seal went out. there was a lot of belt dust around it when i did some clutch work. i also found a ball of belt dust down in the bottom of my motor that i had missed before. it was kind of fluffed up as well. the previous owner had never replaced the motor mounts so im sure he was blowing through belts like mad.

 

[Thunder101]

put 60 miles on sled this weekend when the pull rope broke. started sled with rope around primary and let the sled warm for ten minutes. rode the sled close to 15 miles towards the trail head when the check engine light came on and the sled jerked to a stop. average speed 30-60 miles per hour.

Drain your tank no matter what you think was the problem and when you do that make sure the very bottom (every drop) comes out and goes into a glass jar so you can have a good look at the fuel.
I have seen lots of water in tanks.!

 

[mikew5945]

vinpie pulled his pistons and believes he may have leaned his machine out. this would make it plausible that my crank seal went out. there was a lot of belt dust around it when i did some clutch work. i also found a ball of belt dust down in the bottom of my motor that i had missed before. it was kind of fluffed up as well. the previous owner had never replaced the motor mounts so im sure he was blowing through belts like mad.

Why did he lean out? If, like you, he added fuel to the tank then you both added extra air through the fuel. Ethanol. If you rebuild it dump the fuel like THUNDER said. Start with good fuel. Good luck.