I also suspect your tank is too high, giving gravity feed to your carb. To compensate for the forced flow of fuel, you have leaned down the low speed mixture to the point that your high speed setting cannot possibly supply enough fuel for full throttle runs. If you cannot lower your fuel tank so center line of tank is even or slightly lower than spray bar of carb, you could either change the engine mount to 90 degree/side mount or you could use a regulator like Cline.