Pioneer P61 failure diagnosis

[108logger]

I got an old Pioneer P61 that was sitting outside for 10 + years. Decided to see if it would run, it did for a short period but the engine lost power (wound down) and saw lost compression.

Pulled apart and found piston scored (scratched) near exhaust port. Bought a new piston, crank seals and base gasket, honed out scratches in bore, rebuilt the carb. Got saw up and running but after a few minutes it would wind down and shut off. This happened multiple times with this rebuild, very similar to what it did when I first got it. used synthetic oil mixed with gas at 40:1.

Saw did same thing again yesterday only this time it lost compression again. Pulled apart and piston and bore are scored in exactly same locations as the previous piston

Pretty sure mixture was good. Only other thing I noticed is that after running for 2-3 minutes under light load, the fuel in the tank would be bubbling and the fuel line going to the carb looked like it had air mixed in with the fuel. Wondering if this caused it to lean out?

I've seen 2 strokes seize and burn pistons due to bad fuel / oil ratio but this doesn't appear to be that. Only other possibility is maybe the cylinder was warped and when it got hot it started rubbing more on that side of cylinder.

Anyone have an idea what would cause this?

Saw has a 36" bar and I put a new custom made chain on it. Good parts saw if someone is looking for parts. I can't justify spending more money on it.

 

[alexcagle]

My spidey senses say tank vent clogged.

 

[alexcagle]

Or, Suss gas

 

[alexcagle]

Check collapsing fuel tank hose.
I've seen them suck shut, but tested good under pressure.

Kinda like my wife...lol

 

[Old2stroke]

How are you at tuning? When you rebuilt the carb, are you sure that you have the high speed mixture (H screw) set rich enough? Should 4-stroke at full throttle with no load and clean out and run crisp ONLY when it starts to cut.

 

[pioneerguy600]

Honing out the cylinder? That would be interesting as far as piston fit with a new piston and rings. When the engine heats up there would likely be a loss of compression due to cylinder expansion but you say the piston is now scored? Any chance you can put up a few pics for us to see the damage?

 

[108logger]

Gas cap vent is clear. Gas was new 91 octane with synthetic oil, hose isn’t collapsing either.

Honed cylinder with a fine stone 3 finger hone just to clean up scratches from first piston failure.

Carb was set to 1 turn out on both hi and low circuits and I adjusted slightly to get it to rev smoothly.

I think I mentioned that it appeared gas would boil in tank. At that point it looked like the fuel running through the line was bubbly, didn’t look normal as it did when I first started up. Even tried running with cap off to ensure no vapor lock in tank.

Weird thing is this same thing happened when I first got saw running without any modifications, no carb rebuild etc. original piston and new piston both rubbed and scored cylinder in same spot.

It definitely started rubbing cylinder as it got hot, then it would die out and start up again. Eventually after rubbing into cylinder several times it lost compression.

With the new piston and rings it went through at least 10 cycles of running and dying out before it lost compression. Original piston ran for about 5 minutes, 1 - 2 cycles and gave it up.

I don’t know the history on this saw. Previous owner parked it outside and let it sit for at least 10 years. Might have been having problems and just put it aside.

I’ll try to add some pictures