your probably to rich. its possible it is starving for fuel and can't make power. look in your manual though and see what the rpm raange is....i think its like 2000-11000.

also, it could partially be your throttle linkage and nothing to do with your motor.

-half open carb is not 50% power.
-set a throttle curve to make it more linear, some people brag saying they can hover at 1/3 or half stick...(sometimes yea, its because they have a lot of power) but sometimes it's their throttle linkage and their half stick is 75% power.

if it's a problem with your engine i would say it is to rich. or, another problem could be a small air leak between our tank and either the carb or the muffler resulting in not enough pressure in your tank to properly deliver fuel at higher rpm. (or a small hole in our tank but you would probably notice it) one other thing to check is the plumbing in your tank. Make sure that the tubing to your clunk didn't streatch and is now rubbing the back of the tank. that could cause fuel delivery problems as well.

i would try

-richening the high end
-replacing all fuel tubing
-if still problems replace tank

then, call horizon.....good luck man! hopefully you get it sorted out