'85 Mariner 40 on 18' Fish Hawk

Hello,

This is my first post here. If I come across as a noob I apologize in advance. I recenlty picked up an 18' Nordic Craft Fish Hawk w a Mariner by Yamaha 40 2cyl 2 stroke longshaft on it. I live in central FL so I typically use it to fish in shallower water so I put the motor up on a man jack plate and raised it 5" thinking it would help performance but I seem to have something else going on because it wont rev past abt 3000 rpm. It'll get up on plane, but just barely and i can't figur out what's wrong. The guy I bought it from told me he rebuilt the carbs and put a new impeller in it. I took the carbs apart and they look pretty clean, but Im not sure he set the carbs up correctly when he did the rebuild. According to him it didnt have revving probs and even had enough power to kneeboard off the back. I ordered the seloc manual for it (hasn't arrived yet) and have been working on cars my whole life so i'm somewhat mechanically inclined but I just cant fig out whats going on. Does anyone know if I could maybe rejet the carbs (or recarb) to squeeze some more power out of the motor or if there are any performance improvements I can do? I just gutted the entire boat and refloored and rebuilt the entire interior as light as possible but maybe this little 40 is just not enough power for the hull. Any suggestions are greatly appreciated!!