25 johnson wont rev under load

though it was just because it was a long shaft but i made a riser so the cavitation plate sits even with the bottom of the boat and that didn't fix the problem, its still a turd even with one person in it. it runs fine in neutral and revs out but in gear it just lugs, feels like it wont break 3-4k rpm, never fully plains. the prop is a 10x13 could that be the problem? i cleaned the carb changed the plugs, and it compression tested 115psi both cylinders. what do you guys think?

Sounds like the prop to me


How about the fuel filter - change that?

i think i figured it out, since my throttle setup is a little uh... "custom", it bypasses the throttle plate under the flywheel, which im pretty sure controls timing, becaues it runs smoother when you rev it from that plate then directly on the carb, and theres wires coming off of it. it would make perfect sense for retarded timing to make it a pig under load. also i think the motor is a commercial engine, anyone know what that means?