My buddy just bought a 1986 242 with a 454 mag and bravo drive. The motor is what I suspect to be a gen V. The fuel pump is on the sea pump. He is only getting 48mph at 4100 rpm WOT. I had a 420hp with bravo drive in my 86 242, I got 62mph at 5000. I assumed his boat with 365hp should get 57mph or so.
His setup:
365hp
Bravo 1.5 ratio
21p Mirage prop
48mph @ 4100rpm

He verified that all 4 barrels of the carb were all the way open. What else should be checked?

How old is his fuel ? I had a similar problem with my boat until I ran out most of the old gas. Filled up with 93 octane...problem solved.

Seems simple but worth a look.

I am hoping a tune up will take care of things. I suggested a new fuel filter, cap/rotors, wires, and plugs. It seems to run well right up to 4100, then more throttle doesn't yield more rpm.

Check timing , make sure its getting full advance. Good Luck!!!

I was thinking that too. What should a 454mag be all in? 34*?

It is rare that you need to replace the wires on boats. See if they are flexible and not worn through. Check timing,

I hope it's minor. My friend is not up for a new motor.

If it was running good when he put it away last year and their is no miss-fire...then the only thing that could have changed while it was sitting up is gasoline quality. If it has 12 month old pump grade ethanol in the tank I would be very suspicious of it's quality.

All of the other suspects, plugs, filters, caps, rotors, wires, etc don't go bad (well maybe a coil froze...but it wouldn't run at all in that case) during lay up.

If there's a 1/4 tank or less...add a bottle of gasoline additive and fill it up with 90 octane Non Ethanol and see how it runs after a few hours.

+2

Check the basics as stated. Other things to check would be compression, cylinder leak test. If the engine has been run in salt water, I would do a cooling system pressure test to see if you are getting rust-thru around the valve guides or elsewhere.

Also, how's the structure of the boat? if it is wet/rotting, then the boat would weight a lot more, which can be the culprit.