Float plane speed uncontrollable

I have several Float planes. Most are free ones. As soon as I touch the throttle most will start racing across the water picking up speed with 0 throttle. My aerosoft twotter, D-18 amphb, Alpha Sim PBY5A (FS9, modified for FSX)work fine. My FSD Porter, Otters, TBeaver all pick up speed the second I start the engine.Would anyone have a clue as to the cause.