What propeller are you running, and at what RPM? You want the engine to turn in the 9,000's when leaned to just rich of peak RPM at full lthrottle. You need a tachometer to get four-stroke engines properly-tuned. If the engine is turning much slower than that, you have too much propeller, and the engine is being overloaded. We've even propped them for near 10,000 RPM and they'll run "all day" like that.