OS .70 FS - Compression problem
 

Hello All,
I have a new OS .70 FS engine, that i purchased a couple of years back, and did a few flights on my GP Piper Cub.
I used a normal fuel with 20% castor oil and the engine performed very well. After flying i emptied the oil and filled in the after run Oil .

After a year, now when i try to turn the prop i find there is no compression ... It does pull in the fuel, and empties it from the muffler, but there is zero compression ! i checked the plug and it was fine ..

Not sure what i need to do .. i know there is something small or something jammed that may be causing this.

Please provide me with some insights as to how can i fix this ..

THe crankcase breather does give a hissing sound .. so maybe that is taking away the compression, or maybe i need to check the valves ?

Thanks to all
-Gaurav

RE: OS .70 FS - Compression problem
 

If you are judging the compression of a four stroke, you must have the throttle open full. You also have to turn the propeller in the direction that the engine runs

RE: OS .70 FS - Compression problem
 

Thanks for replying,
Yes i checked with full throttle, and the prop faces the right way ! I can swing the prop without getting any compression .. ( i know 4 strokes get compression after every other rev ) but this does not get me any compression at all ..

Needless to say without compression it wont start ..

Should i open and check the valves ? Maybe something is making it get stuck ?

RE: OS .70 FS - Compression problem
 

Your camshaft follower is probably seized or gummed up resulting in the valves remaining open.Open the valve case and have a look. If you see no movement of one or both valves, then remove the camshaft case, try moving and oiling it.

RE: OS .70 FS - Compression problem
 

That helped.
I opened the valve cover and one of the valves was stuck up . Using some methenol i got it back to working .
I will start the engine tomorrow and do a flight.
Thanks to all.
-Gaurav

RE: OS .70 FS - Compression problem
 

I will just mention that I bought a used OS70II from a guy at our club who said the engine ran fine when he quit flying it about a year earlier. When I turned the crank there was no compression. I removed the valve cover and then put the engine in a bowl of fuel filled to just cover the engine. After soaking overnight, the compression was just OK. Then after running it, the compression is as good as my Tower Hobbies bought OS70II that I'm flying on my J-3.