My 91FX did not get many runs on it as it too was not responding to high throttle. For about 4 flights, it would only run for about 5 minutes before I would notice that there was no increase in RPMs when given full throttle. I use 15% Power Master and a 15X6 Prop. It idled great, ran well on the ground, and it responded normally to throttle after takeoff. But, at or about 5 minutes, the baffling issue would return.

Because there was a lot of burnt fuel or varnish around the head, it was easy to see why. The head bolts were loose just enough to lose compression. It was after my plane's last flight that I found the reason. The cylinder and piston had blackened a lot. A 24 hour antifreeze bath in a crock pot got rid of most of the black stuff. I have yet to try it out to see if the issue was solved.

I don't know if this is your problem.