2004 honda outboard
I have validated fuel lines are clean. Blown air back into tank with no problem. Can accelerate to high RPMs in neutral but when under load and put in gear I can only get to about 1200 RPMs before it stalls. If I baby the motor and choke it while steadily increasing throttle, I can manage to get it to 2500 RPMs. Under no circumstances have I been able to gun it and get the response it should give.

fuel filter changed with oem
New spark plugs
New fuel lines
New primer bulb
Removed fuel pumps and cleaned them up with seafoam
Clean/ new fuel
sprayed gumout in carbs LOTS of it
opened drain screws on bottom of each carb and filled with gumout and sat over night 2x.

 

I had a similar issue but could not run over 3500rpm. like you i assured it was a fuel issue. I finally took it to a Honda mechanic and all he did was drain the tank at the vapor separator and replace the filter above it. It's under the triangle shaped cover with 3 screws. Was able run wide open at 600rpm fine afterwards until my latest issue. Hope that helps.