I finally have my OS75AX running good. I am now using a Graupner 13.5"x8 prop. This seems to be smooth,quiet, and pulls good vertical. I really never understood why this engine wasn't making power. I cleaned the carb., eliminated the filter and richened it a little. This engine doesn't seem to respond to needle valve changes with RPM changes like most engines. I also hate the knocking noise that all OS 75AXs seem to make, but OS says that is normal. Now it is powerful. I do have to keep adding support to the flimsy belly. This plane does fly great though.