1983 omc 3.8L 6 cylinder runs poor after warming up

my boat is a 1983 Grady White seafarer after the engine warms up the boat runs poor I can barely do 2 miles per hour the engine wants to stall I went through the carburetor. I replaced all the fuel filters.

the cap ,rotor ,the plugs, the spark plug wires . I change the coil I made sure the fuel was good I had the carburetor rebuilt and cleaned and still have the same problem it idles nice when you first start it and then when she get out after half an hour almost on cue the engine runs poor and I can barely make it in if anybody out there had same problem and could share some info on how to fix this issue I would be greatful. thank you. PS I did not change the point....I didn't feel comfortable but I did have the timing set.