DaveB

I recently acquired an O.S. 70 Surpass which starts O.K., but will not reach peak RPM (above 8,000 with 12.5x8 prop). Also after it runs for a minute or 2, will quit running. It can be restarted in a minute or so, but the trouble repeats. Have checked the valves and set the mixture screw to factory setting. Bearings seem O.K. in that the engine turns smoothly and quiet. Compression seems good.

Suspect possible cylinder liner and/or piston problem, but I have not disassembled the engine.

Any comments/suggestions appreciated.

Thanks,

DaveB

Campgems

Sorry Bax, didn't see that this was your forum. I removed the post.

Don

Bax

My Feedback: (11)

The engine symptoms you mention sound like the engine's not getting enough fuel. It's too lean for some reason. You need to check the entire fuel system, and not only to make sure it's free-flowing with no air leaks, but also that it's not getting air agitated into the fuel by the engine's vibration. The tank needs to be insulated from vibration. 8,000 RPM with a 12.5 x 8 prop may be about right. We'll see about 9,000 on an APC 12 x 8. Even though your prop has only 1/2" more diameter, the maker, blade airfoil, and blade profile can make quite a difference. We've seen as much as 1,000 RPM difference on same-size props by different makers.

We'd still check out the fuel system, first, though.

DaveB

Bax,

Thanks for the reply.

The fuel system was really the first thing that was eliminated in that the engine was removed from the plane and run on a test stand with its own tank and fuel lines. The symptoms observed, whether in the plane or on the stand were the same. I will check again, but I am quite certain the problem lies elsewhere.

DaveB

Thomasp168

I'd open the needle valve a little. Remember, the needle valve needs adjustment based on altitude, air pressure, temperature. Factory setting has never been 100% on for me.. open the needle (high speed) about 1/4 turn and see what happens...even better, go about 3/4 of a turn, or until it stays running at full throttle, but very rich. Once it warms up, SLOWLY close the needle until the RPMs peak (you should be able to hear it, no need to tach it). Once you peak, back off about 1/4 turn. This will leave you in good shape when the engine 'unloads' in the air.

Hope this helps you