I have a pair of 454 mpi Mags and one of them seems to hit a wall at about 4200 rpm. I have more throttle left but when I give it to it nothing happens. The other motor runs out to 4800 just fine. I checked the throttle body and it's opening up all the way. Any suggestions on what to look at? Thanks Eric

You could be running out of fuel check you pressure on both motors.

are both of your tach,s working properly,is the boat slower than normal,i have seen tach,s go bad for no appearant reason.

What year? If it's mid 90's ('93 to '96 or '97) with the VST tank mounted at the back of the intake manifold, near the distributor, check the screen on the bottom of the high pressure pump inside the tank for crud build-up. Fairly common issue with that system.

Should I check the fuel pressure at wot or is idle ok?

if you have a rubber line on the return for the fuel take a pair of needle nose vice grips and slowly pinch off the return to increase fuel pressure my regulator went bad and it wouldnt let my boat plane for crap

I replace the regulators on both motors last year. By the way I had the same problem last year it isn't something that developed over the winter.

it definately sounds like a fuel issue

Fuel pressure checks out ok at idle. Also these engines are the or the surpentine belt style without the vst tank.

can you get ahold of a scanner to check things like timing advance and such you have the cool fuel system then