The OS 61FX, the OS 75AX, the OS 91FX, the OS 95 AX, and now the OS 65AX are all about the same
weight and will work on the same planes. You pays your money and take your chances. I had 91FX's
and 95AX's on H9 Twist 60's and they seemed to run cold at partial throttle and 15% nitro. I upped the
nitro to 20% and they straightened out. Changing glo plugs did not make much difference although
some plugs were better than others. The engines were not cowled.