Mercury Thunderbolt 500 possible fuel problem


Aug 15, 2011

I'm new to the forums and I would appreciate some advice from some of you knowledgeable boat enthusiast.
I recently purchased a 1970 starcraft boat that came with a mercury 500 thunderbolt 50 hp outboard engine serial # 9050157 (Belgium) and I can get it to run but only when I force feed fuel through the engine with the hand pump on the hose connected to the gas tank.. it'll run for about a minute or so and then die out as if it were not getting enough fuel, but when I pump more than 3 time the fuel leaks from somewhere. I don't know where to find a fuel pump rebuild kit for it so I don't wanna take it apart and ruin any gaskets I may need to replace or something.. I'm open to any suggestions or alternate work around concoctions anyone can help me with. I would really like to get this boat running so I can get her in the water soon. I got a good deal for it so i would hate to not be able to use it, specially since it is my first boat.

thank you very much
- Freddy -


Chief Petty Officer
Oct 16, 2009
Re: Mercury Thunderbolt 500 possible fuel problem

Do you have the vent open on the tank? Without the vent open, it will do exactly as you describe. If it is open, you may need to clean/rebuild the carbs. I recently did mine on my '77 500 thunderbolt, not a big deal. It was harder to get them removed that the actual carb rebuild. Sierra makes kits for the carbs and fuel pumps. I paid $5.70 for each carb kit and $6.70 for each fuel pump kit. Check Ebay, Crowley Marine,, and Wholesale Marine, someone will have them. If your motor is older, do yourself a favor and replace your float needles too. Older needles were not ethanol compliant and will swell and stick. If you have Tillotson carbs and they have those blue inlets, replace them too, they are $5 each. Those deform over time and will leak once they have been removed, even with a new gasket. If you have fuel leaking from somewhere outside of the motor, I would replace the lines and the ball.