Small Engines (Lawn Mowers, etc.)/Chain Saw Engine

Advertisement

Expert: Eric A. Jones - 10/21/2009


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

Small Engines (Lawn Mowers, etc.)

All Answers

Answers by Expert:

Ask Experts

Volunteer

Eric A. Jones

Expertise

Lawnmower Repair . Certified Master Service Technician from B&S. Have 13 years experience on B&S, Lawn Chief, Weed Eater, Echo, Peerless, Wheel Horse, Snapper, Atlas, MTD, McCulloch, Homelite and many other numerous brands. Specialize in electrical repair.

Experience

Born and raised in the midwest. Started tinkering with engines when I was about 14 on my Suzuki RM-80. I began lawn mower repair at a small hardware store. I knew absolutely nothing. I read lots of repair manuals and met an older fellow who taught me many lessons. I continued working on small engines through high school and paid my way through college working on mowers at the same hardware store. Decided to get away from the midwest and mower repair so I joined the Air Force. I repaired air traffic control electronic equipment and ended up in Hawaii where I got a part time job at Small Engine Clinic. I gained a lot of experience from the Small Engine Clinic and had a blast repairing small engines. I then took the Briggs and Stratton Master Service Technician test and earned my MST. I then traveled to Wisconsin where I attended the factory update training seminar and received formal training. I received a scholarship from the Air Force and am currently pursuing another degree.

©2012 About.com, a part of The New York Times Company. All rights reserved.