454 mag rpm question
 

Ive got a fresh 454 mag in my 26 scarab excel w/alpha drive, 19" three blade turbo prop. Ign set at 34 total, engine has lunati 420 replacement cam w/hyd flat lifters. Stock alum mag intake with resh rebuilt roch carb, and cast exhaust, with fast glas risers. The engine has 22 hrs, has strong and even compression.

Question is- im showing 4300 max on the tach, and about 51 mph on the speedo? Any idea of what to look for here? Im more concerend about the lack of RPM, with such a flat prop.

sounds like you may be down on power just a little, what heads are on the engine...the 19 isn't far off if the drive has a 1.32 gear ratio

Check out the thread 454 losing spark on 4 cylinders. Might answer some questions.

I checked compression both hot and cold and its strong and even on all cyls. The heads are the mag 188's, although I kept the stock rocker arms. Could there be binding with the valve lift on that cam?

Have you checked to make sure your thottle linkage is opening the carb all the way?

The rocker can hit the stud where the slot is. Cam manufacturers make long slot rockers to provide more clearence there. That'll make noise and ruin your cam but I doubt if it's slowing the boat. Did you change the valve springs? Weak valve springs on a new high lift cam means valve float at a lower rpm than normal. Look at the carb. Since it was just changed, it's suspect. Could be the jetting is not right and you're not getting enough fuel at high rpm. I'm always doubtful of rebuilt carbs.