Pedec:
I would richen it up a bit or go to a colder plug. Is it the plug recomended for that engine? If so, more likely a high speed mixture problem. An air leak can also cause lean run problems but show up more with low end problems. Plugged carb filter screen can also cause lean high speed. Might be worth checking the filter. When fuel pump diaphram stiffens up with age can also cause high speed to run lean as it can't pump enough volume.