I am anew owner of a 1990 Baja 250 ES. I just had my 454 Mag. rebuilt by US Engine. They claim that they built it up to 425 HP according to mercruiser specs. I was told that these specs were rated at the prop for an inboard engine. I have a bravo 1 outdrive. I installed an Edelbrock RPM air-gap intake, an Edelbrock 750 CFM marine carb, and IMCO powerflow exhaust headers. Otherwise the engine is stock.
I am not a gearhead, but i did assemble this engine from a long-block. I have learned a lot. I only have about 8 hours on the new engine and I am about to do the first oil change. I am only producing 4600 RPM @WOT. (I have only run it like this for a few short moments). My top speed (GPS & Speedo.) is right at 50 MPH. I have a 23P three blade Mirage prop.
I need at least 400 more RPM and I feel like this should be at least a 60 MPH boat.
Any suggestions and tips would be greatly appreciated. thanks.

A Buddy of mine has a 240 Baja and runs a stock 454 Mag 365 HP with stainless II exhaust ( so not really diffrent ) and he has a 25 pitch 3 blade on a bravo runing 72 MPH @ 5400 RPM on GPS !!!!

Something is not right ??!!!
Cant be 425 HP !!!!!!!!!

First check the basics. Check to see if your getting full advance on the timing, approx 32 dgrees. Also you may need bigger jets in the carb.

That carb probably won't flow quite enough for that engine to reach it's potential, but like mentioned above, first check the total timing under load at 3500rpm and see where you're at.

Stupid question, but what risers are you running?
Timing and carb need to be checked, also check your fuel supply from tank to carb, you may just have a slight restriction (USCG check-valve, stuck tap, small hoses, kink, bad pump, clogged filter etc) which may be limiting your ability to flow enough to make power at that rpm so it feels like your engine hits a wall.

Timing, (check the plugs too), Fuel, Carb.

Good Luck!

Are you still running the STOCK MERC exhaust system on that engine??? If you are, then it is going to KILL your power output---it is too restrictive and you may not be realizing the full potential of the so called 425hp. You need a better breathing aftermarket exhaust system such as a Stainless Marine or GIL system. 4600rpm isn't going to cut it....you should be turning somewhere around a MINIMUM of 5200rpm to 5500rpm.

He's got the IMCO powerflow exhaust manifolds, but I wanted to know if he had the right risers on them. The IMCO's will do great for his set up so long as he's also got the correct risers. Reversion won't be an issue, but back pressure will.

Thanks for all the input guys. (Kaama)-are you sure about the 5200-5500 rpm range? Both mercruiser and US engine have reported a max. of 5000. (Ghostrider)-I'll check the fuel lines. I have a mercruiser spin-on fuel filter and an in-line smaller fuel filter. Maybe there are too many devices between the tank and the carb? As far as the risers, i have the Imco "short" risers. Apparently they make tall ones and short ones. Is this a problem?
I took the boat out in some light chop yesterday (1-2 ft.) I trimmed the motor up more than I have in the past and the tach was reading 5000 RPM! I guess too much hull has been in the water all this time. My speedo only showed about 52-54 MPH. I feel like I should see a little more.

Sounds like you might need a bit more stern lift in your prop? If your ass end is dragging and you trim it up to get the revs out of it, you might need to try something like a Bravo 1 prop (4 blade) 23 pitch? I doubt there's too much between your tank and carb, you just need to make sure it's all good! That merc filter will flow fine. Check the screens in the carb bowl intakes, they might be full of crap or something. (mine were!)
Your risers will suit that motor fine, don't worry about them. I just wanted to make sure you didn't still have the old 90 deg cast iron boat anchors on there!
What 'small' inline filter is it and how small is it? pop it off and try that for a run, see how it goes.

It's all one big process of elimination!

Good Luck!