I have a 76 - 70hp Evinrude that was in storage for several years. When first placed in water the motor ran great after warm up. The second trip when tring to take off would bog if the throttle was raised slowly. If the throttle is quickly shoved into WOT it will hesitate and then accelerate to WOT. The bowl of the carbs are very clean, with new plugs. What could be causing the problem?