I'v been running the boat like this for two years with no problems or issues. I just have low RPM of 4200. So you guys think my problem is related to my carbs not feeding enough fuel?

CARB TO SMALL IF ITS A 750? IF I was you have a engine builder check your valve train to make sure its right, thats a cam guys use to up grade there from hp 500s to 600 plus hp. put the darts back on timimg set at 34 to 36 mine is 36 on my 650hp set up. send carbs to nickerson or CFM tell them what cam and heads intake they will set carbs up. like I went from a isky cam a lil bigger, dart intake,merlin marine hrads 310, dry headers carbs tried three a 850 BG made 637, nickerson 1050 made 650hp last is a custom cfm and intake 670hp. thats with a 509. so you should be at 550hp anyways with the right carb and make sure the cam was degreed you loose hp there. and the merc 454 420hps had 800 holleys with out being on a dyno its harder to get it all. I dynoed my motor every winter we made changes then fine tuned jets out on the water. my 850 was 86/86 and 6.5 pv all around. hope you have not leaned your motor out running like that. oh my red line was 5800. Iwould think you would want to run it about 5600. with a 24 ,25 p prop when done. good luck