Recently purchased 1993 GW with a 2000 Mercruiser 7.4 MPI with less than 200 hours, Boat was little used last two years. Outdrive is Volvo original that was connected to Kad42 diesel. Original 3900rpm using B4 duoprop. Now have B3 duoprop but can only run 3800rpm out of 4000 4400 Mercruiser...