I fly the OS 25FX in a H of B P-51D Mustang and when the engine was new it was very tight. It stayed that way for a dozen or so flights and then loosened up. I also had the tank somewhat high relative to the carb and lowered it by altering the foam padding around the tank. The really cranks now. I run it at peak rpm. It has to be one of the hottest mid 20 engines around. My P-51D is a flip-n-fly w/o rudder control and landing gear and it will flat move.