OS 50 sx two-stroke problem
 

Bax, I'd much appreciate your help,again, please.
This time it's my OS 50 sx. It's nearly 5 years old, although it hasn't had that much use, and is being run on Model Technics 10% nitro fuel [which has 20% oil in it]. It is positioned upside down on the model.

All of a sudden it is 'wavering' at mid to full throttle quite noticeably, and is clearly not at all happy. I've changed all the plumbing tubes [and inside the tank, too], cleaned out the carb and changed the plug [an OS8] and tried different prop sizes/pitches. Nothing seems to work.

Any thoughts, please?

RE: OS 50 sx two-stroke problem
 

Inverted running can cause difficulties. First, though, start out from the basics....very rich idle mixture, very rich high-speed mixture and start the engine and get it to full throttle. Set the high-speed mixture first, and then retard the throttle to work on the idle mixture and RPM.

Surging is usually a sign of richness, and the midranges are usually most affected by adjustments to the idle needle. With an inverted engine, you may find that the tank's too high, which only encourages rich running. If you lean the idle mixture, you may find that you'll have to open the throttle a bit to compensate.

If all else fails, get the engine 'dialed in' when mounted upright on a test stand. The settings will be very, very close to those of an inverted installation.

RE: OS 50 sx two-stroke problem
 

Thanks Bax. A helpful reply, as always.

Cheers
Runway