Small Engines (Lawn Mowers, etc.): Chain Saw Engine, compression gauge, piston ring

Question
Eric,

Thanks for being available for questions.

I have a 42cc Sears 18" chain saw that is only about 3 years old and not much wear.  I have had trouble keeping it running - it stalls at idle and then doesn't start easily.  It's been giving me this kind of a problem for some time.  I took the carb partially apart and cleaned cleaned it.  That didn't seem to help much, but I only got about 5 minutes of sawing when it stopped and would not start.  It turns out that there is not much compression - when holding the starter cord and letting go of the saw, the saw drops.  I think it only develops about 70ps.  The cylinder doesn't look scored too badly when viewed through the exhaust port.

My question is: what could have caused this to happen so abruptly?  I'm sensitive to keeping things clean when the carb is off.  I did run it momentarily when the filter was off, but usually the filter is always on.  Could I have done something?  My experience is limited with 2 cycle engines -  is there anything in the carb that could cause low compression?

Thanks for your help!

Doug

Answer
Always leave the air filter on!  Sears saws are not the highest quality, usually only have one piston ring so when it wears you loose most of the engine compression.  You can run a little richer fuel/oil mix but it won't help that much.  If you use a saw somewhat regularly then I would recommend a better brand of chainsaw.

If you used a compression gauge and only had 70 PSI then the saw needs a new cylinder, piston and compression ring.  I have had some run at 70 PSI but they were had to start, would not idle and had no power.

Cheaper to replace the saw than the rebuild.

Eric