Need schooling on piston burn outs

[jimdad07]

I just rebuilt an 044 that had a burnt up piston. The piston was scored up pretty hard on the exhaust side.

Before I fired the saw up, I tried to look for the possible causes be cause I don't want the new one to do the same. I thought that the top ring was missing part of it, but it wasn't on closer inspection. The crank bearings did not seem to have any slop in them, I know I need to invest in a vacuum tester to test the seals, the old gasket (cylinder base gasket) was not in the box with the saw so I could not rule it out. I started the saw up and it runs a little rich with the settings that were already there, but not to the point of nothing but blue exhaust, just a little smoke. I do need to further adjust the idle and get the transition from idle to WOT smooth, it bogs pretty good right now. Also checked the summer to cold running setting, it was set on summer so that none of the cylinder heat is getting to the air intake. Checked the intake boot for cracks, that appears to be as it should. Any other ideas? I just don't want to take out the work I have into the saw by not covering all the bases. BTW, the corrosion on the skirt of the piston is from it sitting in a box on a garage floor for the last four or five years.

 

[parrisw]

You really should have it pressure tested for air leaks.

 

[jimdad07]

I have never pressure tested a saw yet, that is on my bucket list for sure. What do I need and how do I go about testing the saw? I want to do it right for sure. I am getting a lot of the simple repairs like gas lines, coils and such down, now I want to get the rebuilds down pat. I may want to fix some here and there on the side. I will take any lessons you want to give.

 

[mtngun]

See Four Paw's thread on vacuum testing.

Besides an air leak, a melt down can also be caused by a restriction in the fuel supply. Clogged filter, clogged inlet screen, or a tear in the fuel lines.

It may be just a case of straight gassing, and you'll find nothing wrong, but it's prudent to give it a good going over, just in case.

 

[jimdad07]

See Four Paw's thread on vacuum testing.

Besides an air leak, a melt down can also be caused by a restriction in the fuel supply. Clogged filter, clogged inlet screen, or a tear in the fuel lines.

It may be just a case of straight gassing, and you'll find nothing wrong, but it's prudent to give it a good going over, just in case.

Thanks for the information, I am pretty sure I can tackle that job. Very good thread.

 

[JayB]

I downloaded the pic for an enlarged look, but still can't tell. Is that corrosion on the bottom of the skirt? If so, water through the exhaust may have corroded the piston, then running it deposited piston material on the cylinder leading to the problem. You still need to check everything, but if water was involved, I'd also be concerned about bearings in the bottom end. Just a thought...

 

[jimdad07]

I downloaded the pic for an enlarged look, but still can't tell. Is that corrosion on the bottom of the skirt? If so, water through the exhaust may have corroded the piston, then running it deposited piston material on the cylinder leading to the problem. You still need to check everything, but if water was involved, I'd also be concerned about bearings in the bottom end. Just a thought...

The guy who gave me the saw said it happened from sitting on his garage floor for the last few years, it was actually just a scored piston without the corrosion when he took it apart. The guy gave me the saw in a box full of parts for free. I have around $70 into it right now, but I am willing to go the extra mile to keep her running right. It has been a good first major rebuild for me and a good model to do it with, there are a lot of parts out there for it. It is a later model 044 with the 12mm wrist pin.

 

[jimdad07]

Another thing I forgot to mention was that there was only one little spot of aluminum on the cylinder wall, I mean a spot about the size of a pen dot. I was able to scrape it off with my finger nail. I am definitely going to do a pressure test on the saw, just curious for future rebuilds. Anybody mind telling me what you can learn from the burnt up piston itself after the burnout, i.e. the scoring on the exhaust side vs. the intake side.

 

[mtngun]

the scoring on the exhaust side vs. the intake side.

Exhaust side scoring points to excessive heat or a lean mix or straight gas.

Intake side points to sawdust poisoning, although the resultant build up of gunk may also scuff or pit the exhaust side.

 

[sefh3]

How did the fins look on the exhaust side of the cylinder? If they where plug it could cause the saw to overheat. I would look towards a fuel line or impulse line leaking. I'm working on a 290 that was scored but shut down in time to save the cylinder and the impulse line and fuel line where cracked and this caused the melt down. In my signature there is a link to how to build a vac/pressure tester.

 

[jimdad07]

The fins were clean as a whistle, that's not to say that they weren't dirty, the saw was already in pieces when he gave it to me.