I purchased a 75 ax for my 60 sized trainer after original engine 65 LA was damaged in a crash. engine has been ran about 10 flights and ran great but now im having some problems at half throttle engine starts to break down and will start to stutter if you go above half throttle engine smooths out and at idle runs fine engine also transitions from idle to full throttle great . when air plane is inthe air it will actually go from half throttle to the point of stalling then come back
and then i get scared and land because i just learned to fly and I like to land under power. Several people that have been in this hobby for a long time have tried to help me and adjust the needle valve and the mixture valve but i t doesnt make any difference I took the engine apart and there is no piston or cylinder damage I packed foam around the fuel tank to make sure that there was no foaming in the tank. I am running a smaller prop than reccomended in the manual 12x8 because of ground clearance issues but i flew it with this prop succesfully the first 10 flights im using a 15% nittro fuel