Not a go-fast boat but there's a huge wealth of knowledge here. I'm finally tucking my tail between my legs and needing to ask for some help on this one.

Boat: 1994 Wellcraft Martinique 2700, merc 454, bravo 2, 450hrs. Carb is a carter/AFB


Symptoms: The boat feels like its not making any power throughout the entire rpm range right now. It will not get on plane unless light on crew and on a long smooth run. Sits right at 3700-3900 RPM. When the boat does get on plane it revs up like normal since its under lighter load. Pull back on the throttle again and same thing, no power, low revs and has a hard time getting back on plane.

The boat worked flawlessly at the end of last season, popped right up on plane. I rebuilt the carb this spring myself with an OEM merc kit as it hadn't been touched in its whole 23 years and I replaced the exhaust risers. Other than that, there's no new parts. Beyond the low power issue it runs great. Smooth idle, no misfires or backfire anywhere in the RPM range

Its almost painfully obvious that the carb is the issue but I cant seem to let myself believe so.

Here's what I've checked so far:
-floats were adjusted properly on the rebuild
-idle screws have been adjusted
-choke opens when motor is warm
-accelerator pump has been adjusted both leaner and richer with no change in performance
-verified secondaries open with the flame arrestor off looking down the carb and revving
-fuel pressure @7psi on outlet of pump cranking it over, haven't checked running
-has a brand new fuel filter
-tank pickup is clear
-anti siphon valve functions normal
-checked my flappers, no other exhaust restrictions
-engine has good compression

I'm at a loss where to go from here and do not want to start throwing parts at the thing hoping it works.

Are the metering rods in correctly with the springs? Did the jets get switched front to back?

Metering rods are in correctly and jets SHOULD be correct. I meticulously laid everything out when rebuilding, but there's obviously a problem here so I cant rule that out. I need to pull the air horn back off the carb and have a look at everything again. Just trying to find some direction what to be looking for.

Make sure the throttle lever is opening the butterflies all the way.
Any chance the fuel level changed ?

as tink said,fuel level may be low,taking the top off should show the level.

throttle lever is opening all the way. had a throttle cable snap last week and had the pleasure of replacing it. Been one of those years.

Taking the air horn off is going to be my next step in diagnosis.

Also had someone suggest checking timing and making sure it's advancing. Possible sticking dizzy weights. Possible, yes. My bets are on a fuel problem and the carb is the only thing that's been messed with.

Merc T Bolt dist's don't have weights. Unless you installed a MSD dist for some reason.

Good to know. I feel like a noob. Never owned anything that wasn't fuel injection besides lawn equipment

There is a weighted air door over the throttle blades on the secondaries that opens/closes off airflow depending on engine rpm/load/etc. If these don't open or open enough they will cut off airflow to the secondaries and you will have no nutz as a result.
edit in: You will not see this open with engine not running, nor on the trailer with engine running. It needs to be under load...ie: on the water and driving.

Go back thru your instruction book/pamphlet and mentally see if you have messed something up there.

Against common dumb myth - you absoltely need proper secondary operation to get a boat to plane well, if at all. I have proved this live in boat to way too many people.

PS: Don't feel bad. Have worked with several younger guys that are EFi wiz's : construct their own efi, do their own programming, are very good at diagnosing factory and aftermarket EFi issues, etc,etc but yet when I pull a carb apart on the bench they fall over and pass out.

#42 is the air doors. With engine off, you can check by pushing down on them if binding.