I have a 1985 35hp Mercury outboard on a small fishing boat. Most of the time the motor runs perfect. Occasionally I will be on the water running and the motor will die. You can pump the ball but it will never get completely filled with gas. To keep the motor going I have to keep pumping on the ball while running. I have rebuilt the carb, replaced the hose, tried two gas tanks,replaced the fuel pump diaphram and still the same thing. Again, I can go out for days or weeks and it will not happen again. It's almost as if the ball gets an air lock in it. Any ideas on how this happens and how to prevent it?