I am trying to break in my OS 300 twin. The instructions say to adjust the engine to run rich at full throttle to 6000 rpm with the needle valve. Then using only the needle valve,richen the mixture to lower the rpm to 4000. I tried adjusting the needle valve but the rpm remained about the same. I'd hate to ruin the engine by not breaking it in properly. Comments or suggestions?

Basically, you run the engine at full throttle as rich as you can get it without quitting or without the needle valve falling out of the carb. If you can open it some more and the engine's still running, you should be OK.

If the RPM won't drop below 6,000, though, then the engine's not getting good fuel feed. You may have a too-small fuel line or too-small fittings in the fuel tank. This is a 54 cc engine, after all. If fuel flow isn't enough, you can't richen the engine as the instructions state.