1978 Evinrude 85hp outboard. I go out Sat morning to do some fishing and the motor is running great. After a few hours of fishing and running at high and low speeds the motor seems to be missing. The tank is vented and the bulb is firm. The motor seems to be bogging down and then surges for a second and them bogs again. After being in shallow water and tilting up the motor I put it down and the motor seems to run great again, but the sluggishness soon returns. I go out Sun morning but I can only stay out for about an hour and the engine runs great. Could something be wrong with the carbs? Thanks in advance. Joe