I have a 1992 Chaparral 1800SLC with a 4.3 volvo penta and a DP-c duo prop. My problem is that the engine rpm's starts to rise over 4800 and i have to throttle back a bit to keep it in the safe range. With a full load (full tank of gas and 4-5 people) it will top out at 4800 rpm's at 50mph but...