Most lean runs just result in the engine overheating and shutting down. Sometimes though it can cause the plating on the cylinder liner to peel, which will make the engine unreliable. To tune correctly, lean the needle until you can hear a slight increase in RPM when you pinch the fuel line. To test the HSN setting, point the nose of the plane up with the tank about half full. If it will run for about 20 seconds or so, you're good. Then get the low end set so the engine will idle for at least 30 seconds without the rpm changing. That should also give you reliable transitions from idle to full throttle.