My 51 Rapide is experiencing a hesitation and power loss when accelerating in 3 or 4th gear. I've checked the magneto timing, valve adjustments, carb. jets, slides and tuning. Fuel flow is adequete to the carbs and all the fuel screens are clean. The ATD seems fine and the points are good. The bike runs fine up to about 40 mph then hesitates or bucks slightly above that. I removed the air filters and it got worse so I suspect fuel. I also reduced the front cyl.'s spark plug gap to .015" to see if that improved anything, it did'nt. It does not labor or conk out and it gets no worse or better when warmed up. Please advise.