I have a new OS 91FX that I have been unable to get running properly. I have the engine mounted on a test bench. No problems with the fuel system. I set the high speed needle on the rich side – good smoke from the exhaust, and just starting to transition to 2-cycle. The engine will run for anywhere from 20 seconds to a minute, and then just stop. I know the test prop is on the small side (13x6), but I am not tweaking for RPM I’m just trying to get the engine to run. I have not actually measured the head temperature, but it seems overly hot for the short run time. It is not running lean, and there are no bubbles in the fuel line to the carburetor. The engine sounds strange when I increase to full throttle - I was told it was detonating too early. I have been reading the posts here, and I started to dismiss the issue of exhaust back-pressure due to wrong/improper after-market muffler - I am using the muffler furnished with the engine. I decided to take it apart and look inside anyway. The internal cone-shaped baffle was installed backwards – cone pointing toward the exhaust port. I removed the baffle, and will test the engine next week. I suspect this will have cured my problems, but would appreciate comments as to how the baffle direction would affect operation.

Bill Vassallo