OS91 4 Stroke Compession
 

I acquired from a friend a older OS 91 4 stroke. When I turn the prop by hand or starter there is no compression. It won't start unless I use a starter and put my finger over the muffler opening to get it started. Once I have done that it seems to run fine, power seems to OK.
When I shut the engine off still no compression, if I put the starter back on the engine immediately it starts right up without putting my thumb or finger over the muffler opening. The valves are in adjustment, the liner looks OK, and the bearings seem to be smooth.
Any suggestions what could be wrong?

RE: OS91 4 Stroke Compession
 

A low compression feel is usually normal for four-stroke engines, but if you flip them backwards (no glow plug battery attached), they should have a good compression feel. If your engine has zero compression feel when hand-flipping it in any direction, then you have a leaking piston ring or leaking valves. If a bit of fuel squirted into the cylinder brings the compression, then you may be OK.

The engine will run when it's spun fast enough because the piston is moving too fast for the fuel/air mixture inside to get out, and the resulting compression will push the ring against the cylinder wall, and will push the valves closed. The firing process will also do this.

If the engine runs well, has a good idle and throttle up/down response, just fly and enjoy the engine. When the idle starts to become unreliable, then it's time to rebuild.

RE: OS91 4 Stroke Compession
 

I flew the engine today. Seems to have plenty of power. Had to work the low idle to get a decent idle. I have to run the idle a little fast, but I can still land it OK.
Still no compression turning the prop either way.
Thanks for the previous reply.