I have an OS 46FX on my flying lawnmower and all summer it has been running great. I use an APC 11-5 prop on it. One day it would not peak in RPM. I tried everything; high needle valve, low needle valve, new plug, another new plug, more adjustments. Nothing worked. After a weeks of thinking about it I decided on less prop since the field temperature was about 50 degrees colder than in the summer. Put on a Mac 10-6. The engine peaks again and the mower flies about the same as it always has. The key was not being able to peak it on the ground. The engine was over proped due to the colder heavier air.