skypiratescotty,

Using your fingers, try to rotate the fuel nipple (banjo fitting) on the remote needle valve on the back of the engine. If you can move it with your fingers it is too loose and will cause erratic and lean running. Torque the need valve body (not the needle valve) tightly against the fitting until you can no longer move it with your finger tips. That little fix took me from rotten disappointment to total satisfaction with OS engines.

I'm running two OS 91FX's, one with a Bisson Pitts muffler and one with the stock muffler and they both run reliably.

I have two OS 91FX's and two 160 FX's and two of these four engines came from the factory with loose fuel inlet nipple fittings.