I ran into some problems with my 91FX at first but it was not bubbles in the line it was how hot the motor ran in an uncowled plane and the vibration at idle was very bad...the worst I have ever seen in an OS. I finally got the motor to run real good but I had to prop it down to a MS 13/6. I am burning Omega 15% and I think the motor may be alright except for the violent shaking at idle,,,plus it has some fairly bad top end vibration also. The low speed vibration was so bad it backed out 2 of the bolts that secure it to the plastic motor mount. When I tried to tighten the bolts they just stipped the threars in the plastic mount because the plastic was so hot.
The really strange thing is my buddy has the same motor with stock muffler as do I and he is swinging an APC 14/10 and it pulls real strong and no bubbles but his also runs hot and has a lot of vibration also. I know that there is going to be more heat than the 61FX will have because of the larger displacement in the same size motor but this heat and vibration thing is really going to be a problem. I am going to balance the crank on mine and see if I can get the vibration down...I did this on a Super Tiger 90 that had a bad vibration and I was able to get the vibes down to near nothing or as good as it can get for a single cylinder.