Sounds like you have either old fuel in the tank, wrong fuel/air mixture, or plugged carberator jets. First thing that I would try is to adjust the fuel/air mixture. Near the top of the carb on the front or the side there is a slotted screw. Turn the screw all the way in until it seats. Then turn it counter clockwise 1 1/2 turns. That is your start point. Then with the engine at an idle in gear slowly turn the screw clockwise until you hear the engine start to die. When it when it sounds like it is going to die turn back out 1/2 turn and it should smooth out. this is your correct mixture and should be readjusted anytime you rebuild the carb or run at a different altitude. Second thing that I would try is while running engine get the RPM's all the way up and while still maxed out pull the choke knob out and when it is about to die push knob back in. That will cause a vaccuum inside of the carb and possibly clear jets out. If that doesnt work then I would pull the fuel bowl off of the carb and make sure it smells like fresh fuel and not old fuel. Old fuel has a very distinct smell. if the fuel smells old then you need to have the carb rebuilt and that should be done by an outboard mechanic.