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.