I have a 1987 OMC 4.3L cobra in a 22 foot Sea Ray Seville mid cabin. Here's my problem. I have a 13 pitch by 15 1/2" and a 17 pitch by 15". Neither prop will tach out the motor, the best I can get is 3700 RPM with the 15 and 3600 with the 13. Motor starts, idles, and runs great. Any...