If tuning, especially richening up the high speed needle doesn't help, check the carb screen and then the fuel pump diaphram. I have seen the pump diaphrams get distended and then they won't pump enough fuel at high throttle. Also verify that your pulse line to the fuel pump is in good condition with no kinks nor air leaks. Ideally, that should be a somewhat stiff line, and as short as possible.

(All this is assuming you have no air leaks whatsoever in the fuel supply lines.)

Also, do you have a filter in the fuel tank? Could it be partially clogged?

AV8TOR