im saying its the fuel delivery. at high rpm you use more fuel than the lines can deliver... cure? pull the tap out of the tank...take off the poxy lil filters there. replace tap. then use a "hi-flow" inline filter after the tap. and maybe, very carefully, adjust the carbie floats so that the fuels a little higher in the bowl. check fuel lines arent kinked or pinched.
and valve clearance? 4 thou, or 0.004 is pretty normal. or about 0.1 mm... two sheets of paper roughly.