The Chainsaw Teardown

I have been tearing down and diagnosing my bro-in-law's chainsaw from a catastrophic failure.  It appears the piston welded itself to the cylinder wall.  This is an MS290 "Farm Boss" only 6 years old or so.  Tearing it down, I see evidence of a few problems contributing to the overall failure.

Tearing off the muffler, it is easily seen the piston is severely scored, and a piston ring is smushed.  This is bad.  After a little research, the most likely cause is either no oil in the gas, or old gas mix.

It's also possible to get this problem with an air leak in the fuel pickup or the carburetor boot, causing the engine to run lean and/or over-rev.

I took off about 300 parts to get down to this point.  That cylinder sure was difficult to pull off.  It took quite a bit of effort.

It's hard to see in the picture, but it's easy to feel with a finger:  The cylinder is trash due to heavy scratches.  It wouldn't hold compression if only the piston was replaced.

The piston is so small compared to the big antique John Deere tractors.  This piston measures at 46mm, and idles at 2,800rpm with a top end of around 12,000 rpm.

Dirt restricts air flow in the air-cooled engine.  

The dirt and grime surely didn't help in this matter.

One interesting find is this:  The race that holds the balls of the ball bearing in place is plastic.  It was broken in many places, but all the balls were still in place (at least for a little while).

The air filter was pretty well plugged up with a quarter inch of fine dust.  This can lead to engine problems from what I've read.

I squirted some degreaser on the main parts, and hosed them down.  

It's amazing things cleaned up so nicely.

I'm expecting a cylinder/piston rebuild kit to arrive in the mail Tuesday.  The kit I ordered bumps up the engine size from 46mm to 49mm.  Thus, it changes it from a MS290 to a MS390.  This bumps up the power from 3.8 bhp to 4.3 bhp or about 13%!

I've also ordered a few other parts (fuel line, impulse line, new air filter, and a missing chain bumper) off of Ebay.  Those are coming from Hong Kong, and should arrive by November!  Geesh!