So just bought this boat it’s a 99 rinker 232 boat idles just fine in neutral can rev up rpms without a problem but when you put it in gear and under a load at lower rpms it will backfire at wot the backfiring stops but boat just won’t build rpms and just falls on face. Has new distributor cap, new plugs wires already traced plug wires to make sure they were correct, new plugs and gapped properly, and new fuel water separator not heard a million things it could be but just not sure exactly what it is. Any help would be greatly appreciated.

Hook a pressure gauge to the shrader and see what the fuel pressure is doing.

we did hook a pressure gauge to it just not sure what the psi should be at it seemed ok but want to try it under a load and see what it does

New rotor with cap?

Just the new cap as far as I know

Place motor in base timing mode set 8* BTDC
Plugs MR43T gap .035
Fuel pressure 43 psi
WOT 4200-4600

Check for carbon tracks on rotor if not replaced. (just put new one in). Like AD said, verify timing, check fuel psi under load.

Update just did a fuel pressure test at idle due to weather and work haven’t been able to take it out and put it under a load for a fuel pressure test but at idle it was at 70 psi I don’t know a lot about this motor but from what I have read that’s a tad bit to high.

Yes that's high, your pump is fine, but regulator (item 15) isn't working or line is all clogged up going back to the fuel filter

https://www.mercruiserparts.com/bam/...31918/2705/110

Not sure if it is even possible in a marine system but it almost sounds like what you get with an exhaust restriction in a car or truck. Also, how about crappy fuel since you just bought it ? How long has it been sitting ?