OS .35 FP reving up after a while.
I'm flying a Top FLite Tutor I profile with a OS .35 FP spinning a 10X5 Top Flite wood propeller. I set up my engine running rich on the ground (about 8500 rpm's). After about 2 1/2 minutes of flying time, the engine starts reving up and remains running very fast for the remaining to the tank. I'm using a 4 OZ uniflow brodak tank.
Any suggestions???