I have an os 91 fx which isnt running steady. Is easy to start , but when it runs, the rpm is unsteady. This is first time Ive experienced this type of "unsteadiness" (maybe bad english - Im from Norway ). Its difficult to explain, but I will try:

The sound from the engine isnt steady, one way to tell itis that itis 2 different rpm-levels at almost any throttle-level, not at full throttle. When the engine runs, the engine is jumping between thesetwo rpm-levelsin a steady frequence. The highest rpm-periodin the frequenceis always longer than the lowest. Both the rpms is "following" each other propotionially when I change the throttle.

Maybe difficult to understand what I mean

Last summer, I crashed the plane with that engine, an oldpatternplane, using the standard muffler included with the engine, in the next plane, a goldberg cub, im using a pitts muffler. Maybe that will help

This could be somedirt in the needle valve seat, but I have checked this.

Someone havesomeknowledge about this type of engine problem?

Maybe O-rings and air-leakage?