02 225 ox66, can't reach full rpm's

[McGillicuddy]

I assume you are running a 23'. First check your tach that it is set for a 6 cylinder motor rather than 4.
Then measure the height of your motor. Anti-cavitation plate should sit at least 1-1/2"above the keel while the two are parallel.
Finally what prop (specific as possible... make, model, diameter, and pitch) are you using. Sounds like your prop has way too much cup and rake, or the motor is way too low, or the tach is off.