John Deere 21C trimmer no compression

[dfs]

John Deere 21C string trimmer M0021CX sn 067417. Does not start after winter. Has been problematic thru the years. New gas, oil & spark plug, no go. Can feel spark is present. Hold my thumb over plug port and rotate fan - very low pressure, not good. Soaking top of piston with penetrating oil. Any other ideas? Or time to replace? Thank you.

[KE4AVB]

Very low compression. It sounds like the piston ring is struck. You will need disassemble the engine by separating the cylinder from crankcase. You need manually clean the piston ring and groove; that is if you don't break the ring trying to get it loose. While there make you piston and cylinder is in good condition or you wasting your time. You will need at 100 psi for it to start.

Note: Cylinder to crankcase gasket is not available separately you must order gasket kit AM115932. If you should need to rebuild the carburetor the ZAMA kit is RB-25.

[dfs]

Thank you for the reply. I went deeper - took carb and 'spacer' block off of cylinder head. I can just see a clean looking piston ring when cylinder is at bottom of stroke. IF the original cross section of the ring was a round "O", it is now a polished "D". Near the top of the cycle, I can easily blow air pressure back through the spark port and out the carb port. I don't give up on stuff if it can be restored, but it looks like the time has come. Thank you again, glad I re-found this forum after the PERR board seemed to disappear.

[KE4AVB]

Just looking at it isn't going tell you if it is stuck or not. Your going need to see if it will move any and it a lot easier to get to through the exhaust port.

BTW the piston rings is usually rectangle shaped with some thin and others wider just like larger 4 cycle engines. The main difference is there is most times an anti-rotation pin so the ends of the ring does doesn't hang in the ports.

[NO0C]

The main difference is there is most times an anti-rotation pin so the ends of the ring does hang in the ports.

Don't you mean does not hang (line up) with the ports?

[KE4AVB]

Yelp, I change that. Customer came in the door as I was typing it up.