Gentlemen, I have a problem that I hope none of you ever experience. I have a 7.4L, 23 ft Fourwinns that is giving me fits. She starts, idles, comes up on plane perfectly, but when I try to accellerate beyond 25mph she starts popping off like a kid who was told to clean their room. Sound like back firing, very noticable loss of power. As soon as I back out of the accelerator she quiets down and runs just fine, so long as I don't try to go faster than ~25mph.

Gas is new/good, plugs are all tan/gapped/dry. Looking for suggestions as to where to look next.

what yr engine? sounds like timeing, may need to be advanced or retarded. do you have a timeing light?

also gm's used to use a spring advance in the distributor, these can stretch and or break. easy fix if you know your way around a dist.
if its a newer engine then I may not be much help as I don't know that much about all the new junk on them.

What year? Fuel injected or carb? Had a similar problem on one it was the high pressure fuel pump it was supplying plenty of fuel just not enough pressure to run WOT

4 brl Carburated, and the engine is a "97".

Funny thing is she draws a lot of gas, and RPM is high while coming up on plane.

Also, if the timing were off, would she not sputter/squwak across the entire powerband?

not if its not advancing right, the more rpm's the further the timing has to advance. as you pass a certain rpm the timing will become off and then the sputtering starts.
like tommy said too the fuel pressure might be the culprit, have you changed the filters lately, check for fuel lines clogged/deteriorated due to ethanol. if its able to turn high rpm's in neutral I would start with timing first.

I'm going to take a marine mechanic for a ride Saturday morning to see what he thinks. I'm hopping that I have cracked insulation on some ignition wiring that archs at higher rpms. I really don't care what the problem turns out to be..., so long as I can fix it!

Gosh, I sure do wish I had kept my pontoon boat.