checking fuel pump

[maineiac5586]

I have a 76 merc 500 and want to know if fuel pump is bad. it had fuel pump built into side of carb body with the little blue fuel inlet caps that say tillotson. i blew into the fuel inlet and can hear air rushing into carb until i raise float, then i can't blow anymore. my fuel line bulb get nice and firm when squeezed. any other way to check the fuel pump to see if needs to be replaced? i notice when i squeeze the bulb it cures my bogging but i also noticed the top carb float level was set too low. don't know if float level is cause or fuel pump. i like to diagnose correctly and not just throw parts at it. thanks

 

[hkeiner]

Re: checking fuel pump

If adjusting the carb floats to spec does not solve the problem, then rebuilding the fuel pump would be the next thing I would suggest. Rebuilding the fuel pump can be considered routine maintenance if it has not been done for several years. Also, the cost is not much if you do the work yourself. It would not really be like throwing parts at a problem.

You can test the fuel pump with a vacuum gauge connected to the fuel line upsteam of the fuel pump (using a T connection), but you kind of need to know what to look for. In general, you would run the motor at idle and briefly clamp the hose shut upstream of the gauge and see if the vacuum goes high enough. I don't know the spec of your motor so I can't say how high the vacuum should go to indicate an OK fuel pump. Perhaps your shop manual has this information.

Check the link below for the Mercury Service Advisory on how to test a fuel pump with a vacuum gauge:

http://forums.iboats.com/showthread.php?t=300241&highlight=vacuum