It might be too lean. Have you tried holding the plane with its nose straight up and giving it full throttle? (You should have a friend help as a 60 is pretty powerful and you shouldn't try to hold it with one hand while advancing the throttle on your radio with the other!). If it leans out try opening the needle a bit. Try to figure out under what conditions does it stall? In particular, what attitude is the plane on? Climbing? flying level? How are you advancing the throttle? Slowly? Or "slamming it" open? Can you determine if there is a "dead spot" in the throttle advance? Does it only happen when the engine is cold? Hot? Either flying or on the ground how much smoke is there? What does the oil coming out look like? Make sure the clunk isn't stuck someplace. Be aware that it willl fly differently (under load) than on the ground or test stand. I'm no expert, but the answer to these should provide some help. I'm not very experienced in r/c motors but these are always the things the "guys in the know" at my club want analyzed when I have a problem. I am pretty knowledgable in 1:1 scale (auto) motors, including port timed 2 strokes (which is what this motor is). Although these are pretty simple, they are very high performance relative to displacement and therefore are pretty sensitive to tuning and fuel/air flow.