I just had my HP 500 rebuilt from a reputable engine builder in my area.The entire engine was gone through will all new parts,cam,pistons, rings,etc.The engine was tuned and run on the dyno.The peak torque was 592 ft.lbs.from 3800 rpm's to 4600 rpm's.The peak horsepower was 566 @ 5600 rpm's.The #'s looked good on the dyno.I have a 95 27 foot Fountain.The boat ran 68 gps @4600!!WTF!!The boat ran 68 last year with what turned out to be a broken spring running on 7 cylinders.When the motor was at its prime prior to it being rebuilt,it would run 74-75 all day everyday.My engine builder is scratching his not knowing what could possibly be the problem. ANY HELP OR SUGGESTIONS WOULD BE GREATLY APPRECIATED!!!

It's only running on 7 cylinders.

Interested to hear input on this

It was before i had the motor rebuilt.

I am running a brand new hydromotive 25 pitch prop.I was spinning a 27 pitch mach prop before the rebuild.

What rpm are you spinning the new prop?
Have you verified the ignition is good on all 8 cylinders?
Have you done a compression test on all 8 cylinder? (Rebuilt or not)

Verify good fuel pressure when wide open?
Timing?

Agree^^^^^

Basics first.

Compression and fuel psi....fuel psi at WOT too and especially.

Carb could be out of whack....sitting does strange things to them.

Timing ?

And then....what cam was used in it ? The HP500 (Crane '721) ? Or ????

I am spinning the new prop @ 4600 rpm's.The prop is not the issue.I had them replace my msd ignition box with a new one and the distributor as well.I have not done a compression check again since the rebuilt.Fuel pressure was checked at idle and not at open throttle yet.The timing is at 34 all in.

The carb was rebuilt.I can't remember the brand for the cam but it's definitely more aggressive.