I agree with you Daniel...I was very disapointed with my 91. I am not sure this was such a good idea for OS to put this engine into production. I was able to get mine to run fairly good but the vibration at idle that it shook my plane so bad the nose wheel was a blur and it full throttle it was vibrating so bad it backed out 2 or the motor mount bolts(8-32 bolts) and the motor ran so hot that when I tryed to tighten the bolts the mount was soft from being so hot that it stripped the plastic threads. I guess I got spoiled from OS engines of old where you brake it in for 2 tanks, adjust and go fly.
I am going to balance my crankshaft to see if I can get the low speed vibration down, I have done this before and I had to remove a bunch of steel to get the counter balance on the conrod end of the crankshaft to sit straight across from the counter balance with the piston, conrod, and wrist pin lighter than the counter balance. It worked real good as I was able to eliminate most of the vibration. After I balanced the crankshaft it was the smoothest motor I have ever had, plus I gained 500 rpm's.
I am not real sure what I am going to do to get the motor to run cooler.
I only messed with it for 1 day and when I got home I cured my problem by installing a Super Tiger .61 with a tuned pipe and now I am flying and not fighting. My buddy has a .91FX that runs great and he is swinging an APC 14/10...go figure.