It also sucks water through a different system not the outdrive. What's that called and what's the reason for it? I'm assuming it pumps more water?

A bone stock 454 mag will run mid 60's in that hull with a 23 mirage as stated previously.

take a flashlight and look down between the exhaust manifold and cylinder head. i believe it should have merlin cast into it on top of one of the exhaust ports if that is what they really are.

One way or another 4550 rpm is low unless that is a stock 330 like stated as well.

I had one with a 500hp 496 and it ran low 70's.

Another thing to consider is whether the cam is installed correctly.

On my older baja my bravo 1 had corrosion so I went to a low water pickup just so that I could get enough water to keep the boat cool. Its needed on the big hp motors I gathe for cooling. I only have it because it was easier for me to do that then deal with the corrosion of the lower unit.

Is your engine a 7.4 litre or a 454 mag? Im guessing (based on your top speed) that its the 7.4 litre. If thats correct than your RPM's are spot on and so is your top speed.

I had a 1990 250 with a 7.4L 330hp (not 365 mag). Boat ran bone stock 60-62 trimmed out at around 5000 rpm with a 21p mirage for 10 years. Lost a couple mph after that...

It definitely sounds like the engine is not performing like a 360-400 hp motor. If it's not a tuning thing, propping down to a 21p 3 blade will help you greatly. I still have this prop if you want it...

After the first 10 years, I did a simple upgrade adding an edelbrock performer rpm and edelbrock 750cfm carb - helped the boat start better, but no performance improvements.

The next year, I put lightning headers (no rebuild of the engine) and removed the hook/pads from the back of the boat. I added K-plane 280 tabs for extra stability and rough water performance. With a tired motor, I was getting 65 gps after the header change... I think that worked nicely with the carb/intake changes, not bad for the peanut port heads.

I don't recommend a 4 blade prop unless you made a serious engine/drive change to enhance mid-range. With the depth drive on that boat, you will be able to pull tubes and get best top end with a 3 blade prop.

Good luck!

So from what I'm reading here a stock 454 w/330hp will run low 50's and a 454 mag w/365hp will do mid 60's? So for 35 more hp you get over 10mph? Seems like a huge difference to me.

From what I understand it takes 100+ hp to gain 10 mph. So a 365 would only be roughly 3 mph faster if that 100hp=10mph rule is true.

I couldn't see Merlin cast into it because the exhaust is in the way but I looked around and it does have a high rise dart intake manifold.

Seems really slow for the engine. I had to run my 280 on a single engine once and could get 45 with the one motor turning 4800.

I'm going to change the fuel filter and hope it helps, seems like it would be the smartest thing to do first. Thanks I appreciate the help guys.