Propeller options

[Kahuna]

First, I would like to thank you for allowing me to join your forum. I have recently aquired a 1976, 20' Seafari, the I/O model (boat is clean, always trailered, pretty sweet ride). The motor and outdrive combination currently installed, is a 140hp mercruiser (approx 200hrs), coupled to an Alpha 1 outdrive. Compression on motor checks out very well, and outdrive was completely gone through in the early part of the summer, prior to my taking posession. Both pieces (I/O and motor) came out of a 25' Blackfin with twins (guy took a plastic bag on the intake of other motor, and over heated it,... replaced both engines and outdrives last season). Here in-lies the problem. The propeller that is currently installed is a 13.75 x 21. I'm not getting the RPM's out of the motor. It is supposed to turn 42-4600 wfo, all I can get is about 3850 (I have been through the motor/fuel system, and I know I'm down to a torque issue from the propeller). I'm aware that a single inch of pitch should account for about 200rpm on the top end(this would translate to a 13.75 x 19), but before I go spend $$, I'm sure that somebody has a nearly identical set up they have proven and are happy with and has already been through this evolution. Any suggestions?