M7 engine failure

[kooze21]

Has anyone had any engien issues on the m7? i have blown my engine the last three times i have rode it. It works great in the mountains but around home it blows the left piston. i have noticed all 3 times the piston skirt is breaking. like i said, in the mountains no issues. any ideas as to what it might be. the engine light does not come on but i am thinking the computer is not setting my fuel to low altitude.

 

[moyiesledhead]

M7's are pretty bomb proof. Unless you already replaced the cylinders.......next time you rebuild it check the cylinder bore diameter. May be out of tolerance. Broken skirts are usually caused by piston slap due to too much clearance. Never an issue on my M7, but I've seen it on a couple of 1000's. Also, I run EGT's on every sled I own so I can see what's going on in there. Don't have to guess what the computers doing to your fuel. Also, have you been doing crank seals, or just the top end? I pretty much always do crank seals when I do a top end. Learned the hard way!

 

[kooze21]

i replaced both cylinders and resealed the whole engine. i just can't see why it only blows at home and not in the mountains when i'm working it hard.

 

[moyiesledhead]

Well, that eliminates both my ideas! Not over revving it at home are you? They're usually clutched stock for 6000'. Mine will run 8200 rpm at 3000' if I let it. Not good for them to run over 7800rpm.

 

[kooze21]

no it holds at 7800 nothing over that. i did put a set of v force reeds in, but even if they were the problem it shouldn't matter what altitude i am running at

 

[moyiesledhead]

Nah, reeds shouldn't be the problem.

Hmmmm......I'm out of ideas.

 

[kooze21]

thanks for the replies, i need to do more digging and figure this out. maybe a fuel pessure issue as well.

 

[Pro-Am Chris]

A fuel issue would not cause the skirt to break. If it were running lean you would be burning holes through the top. Sounds like a tolerance issue.

 

[kooze21]

where would the tolerance come from when you have new jugs and new pistons?

 

[catinthehat]

What brand of pistons are you using?
Measure for clearance before assembly to be sure parts are correct as well.

 

[m7 carver]

You guys are saying a M7 should only rev to 7800? What wieghts are you guys running in yours?

 

[ippielb]

Check you crank runout, also measure the cylinder top dead center on either side, make sure they match up.

 

[Pro-Am Chris]

where would the tolerance come from when you have new jugs and new pistons?

In your crank and rods.

 

[OLE 11]

Sounds like the connecting rod bearing to me! If there is any play..up or down...its facked! You should take the bottom end apart when you brake the skirt.... 9 out 10 times there will be chunks of ch!t that fall down into crank!

 

[kooze21]

i've had the bottom end apart and redid it, has any one experienced any mixture issues with the altitude. cause i am starting to lean towards the electronics side of things.

 

[moyiesledhead]

Is this engine stock, or modded?

 

[kooze21]

Stock

 

[Bnorth]

Check your piston to wall clearance.

 

[moyiesledhead]

Check your piston to wall clearance.

Been suggested twice. New jugs and pistons, but should still check it.

 

[moyiesledhead]

Stock

That pretty much eliminates detonation. You only need 87 octane for a stock M7.