Any way to tell if a air leak vs lack of oil caused a saw to burn up?

[crazyal]

It's not my saw nor have I seen a picture yet but a friend asked me why his saw lost compression. He said he pulled the muffler off and can see grooves in the piston and ring that line up with the edge of the exhaust port. He said the saw had been sitting for a few months before he fired it up and thought maybe it could have been oil separation but he had ran it for 10 or 15 minutes before it just died. It would start back up, run for 20 seconds then die again. He did look at the plug and thought it looked like it was running fine, a medium tan color.

I thought about trying to block off the exhaust and air filter and try to pull a vacuum and see if it would hold it. He lives several states away so I don't know much about the saw (brand or size) but I think he said it was only a couple years old with very little time on it. My personal opinion would be as long as there was enough oil in the gas once you start the saw up the vibrations should be enough to quickly mix it back up but I've always made it a point to give the saw and gas can a good shake to make sure everything is mixed well.

 

[MossRoad]

I don't think the oil would separate from the gas in a few months. I've had two stroke engines with fuel in them for longer than that and have never seen a problem with it. And as you said, you start it and its vibrating and off you go. However, many have told me and I have also observed personally when destroying a chainsaw or motorcycle engine, a two stroke engine that is starved of oil (wrong mixture or bad oil pump if equipped) will score the piston up pretty quickly, especially on a cold start, and that's all she wrote. Especially if the engine is run up to high RPMs directly after it fires.

If he said it ran for 10 minutes, then died. Then when he starts it it runs for 20 seconds and dies again, that could be another problem. I just had that same issue with my Poulan and I found that the muffler mounting bolts had worked loose, which caused a leak between the cylinder and muffler. I tightened it back up and its running fine again.

But if if he can see a scored piston, its time to tear it down and see what's what. He may luck out and have no cylinder damage, but I doubt it. New cylinder, piston, rings most likely. Hopefully no chunk of metal got into the bearings and such.

What kind of saw is it. Maybe someone has some experience with that particular saw make and model?

 

[Ed of all trades]

As MossRoad said it sounds like the wrong mix or no mix at all and just gas. Sounds like time for a rebuild if it is a good saw or if it is a cheep homeowner saw, a new one. Ed

 

[LD1]

To answer the question, no i dont think there is any way to tell the difference.

An air leak causes the saw to run lean, which is the same thing as starving it of oil.

So while lack of oil can be caused by a few different things, the outcome is always the same.

 

[crazyal]

I'll try to find out more information and even get a picture or two. I'm not sure how quickly he'll get to it as, unlike me, he doesn't burn much wood so he only uses it to keep a couple of trails clear from blow downs. Until he asked I had never really thought about it. I know my Husky 357xp is a saw that had an issue with some of them where the rubber boot that connects the carb to the cylinder would leak on early production models. I just like to learn so I thought I would ask. Would an air leak usually be between the carb and cylinder or do the seals on the crank shaft also go bad? A leak near the carb could be something that seen when removing the cylinder but the seals would be harder to check. If it ever happened to me, unless I knew for sure that it was a lack of oil, I would want to try and figure it out before dropping a hundred or two on a piston and cylinder only to risk it happening again.

 

[Leejohn]

Piston scoreing and top of piston is ok then it's low or no oil. Top of piston bad then it's lean out, it well also score the piston most of the time.

 

[Sodo]

Oil and gas won't separate. And whatever stratification may occur from sitting certainly won't exist after the starter rope has been pulled once.

I would first suspect that possibly the saw was run with straight gas one time or another, and developed the grooves in the piston from that incident. The ring is probably stuck there. Easy to happen, if it sat long enough to forget, and no rigorous procedures about your gas cans, or lent out the saw?

If the wrong plug (too hot) was used in the saw (at one time) it could burn the piston top and maybe seize the ring. Lots of folks mess with spark plugs because its something that seems easy enough to do. Maybe some damage was done in the past, which failed to heal.

If there definitely ALWAYS has been oil in the gas, then suspect that a carb jet or passageway is partially plugged. Not enough to prevent starting, but was running lean. But plug shoulda showed that. Can you get the fella to look inside the plug hole, see of the piston top is all black -OR- has lighter aluminum color (burned)?

An air leak causes the saw to run lean, which is the same thing as starving it of oil.
So while lack of oil can be caused by a few different things, the outcome is always the same.

LD, I see you may have been around the block with respect to saws. I have read that that too much oil creates a lean condition (a well-oiled, lean condition). That upon atomization the oil and gas separate, where the oil goes in as droplets of pure oil, and don't ignite, don't contribute to the burn. You seem to be saying the opposite.

 

[Leejohn]

Oil and gas won't separate. And whatever stratification may occur from sitting certainly won't exist after the starter rope has been pulled once.

I would first suspect that possibly the saw was run with straight gas one time or another, and developed the grooves in the piston from that incident. The ring is probably stuck there. Easy to happen, if it sat long enough to forget, and no rigorous procedures about your gas cans, or lent out the saw?

If the wrong plug (too hot) was used in the saw (at one time) it could burn the piston top and maybe seize the ring. Lots of folks mess with spark plugs because its something that seems easy enough to do. Maybe some damage was done in the past, which failed to heal.

If there definitely ALWAYS has been oil in the gas, then suspect that a carb jet or passageway is partially plugged. Not enough to prevent starting, but was running lean. But plug shoulda showed that. Can you get the fella to look inside the plug hole, see of the piston top is all black -OR- has lighter aluminum color (burned)?



LD, I see you may have been around the block with respect to saws. I have read that that too much oil creates a lean condition (a well-oiled, lean condition). That upon atomization the oil and gas separate, where the oil goes in as droplets of pure oil, and don't ignite, don't contribute to the burn. You seem to be saying the opposite.

Agree the spark plug and the top of the piston should tell you everthing how the engine is running and what went wrong.

 

[crazyal]

So he sent a couple pictures. The plug is dark tan on the ceramic and black on the metal.

 

[LD1]

Too much oil to cause issues would have to be ALOT too much oil. Like 10:1

Some of my saws call out 32:1
Some call for 50:1

I run 40:1 and worry about nothing