I know what you mean Brett...I just said to hell with it and installed an old but very smooothSuper Tigre .61 on a macs tuned pipe and it is so much fun to have a good running motor . I did not mess around with my OS 91 for very long because it behaved almost as predicted so I just decided I wanted to get some air time and have some fun so on went the ST .61.
I am going to balance the crankshaft on the 91 to see if it will smooth out some plus I have a carb off an old 61 SF to try . I hate to give up on it totally but I am not buying a pump or a needle valve for an engine that vibrates so bad it was shaking my plane to pieces.
While I am unhappy with the .91 I am not going to say I will never buy another O.S. but I will take a harder look at the other motors on the market from now on.