How do you land the plane at half throttle? 4000 RPM? Are you sure you are using the 2 blade setting on the tach rather than the 3 or something? All my engines (three OS .46's, one OS 50, three OS 1.20 AX's and my new OS 75 AX) idle below 2500, and this new OS 75 AX idles perfectly at about 1800.