A too lean mixture setting will eventually damage your engine. An easy way to check if you are too lean is warm your engine up fully and then ride it at full throttle for a little bit. Immediately cut throttle and pull clutch lever in. Engine should immediately drop to idle. If engine speed remains high and takes a second or two to drop to idle, it means your mixture is too lean and you should lower your needle clip by one position and repeat as necessary. If dropping clip doesn't help then check for airleaks.