This can sometimes be down to the throttle rods being wrongly adjusted, especially the vertical one. If your pedal gives you full throttle before it reaches the floor, adjust the vertical rod until full throttle is "pedal to the metal". When fully to the floor it shouldn't put any strain on the mechanism at the carbs but still give you fully open butterflies. You'll probably need a small adjustment to the central push rod to the carbs. There should be a fraction of slack there when idling. Lever should be on the stop, not floating.