Heres the story. I have a '81 F100, with a '75 351W. I finally decided to get rid of the temporary electric fuel pump and put on a mechanical. It took me a long while to even get fuel to the carb, but when I finally got it running, it ran terrible. And I noticed a river of fuel being dumped into the secondaries and some leaking out the side. I know for a fact that it wasnt doing this before I swapped pumps. What did I do????

 

Check and see what fuel pressure the mech pump is rated at/ putting out. If you're running a Carter / Edelbrock carb, they usually only tolerate around 5 psi. Some mech pumps put out 9 or 10 psi.

 

Im running a Holley 600cfm. Im not sure exactly the psi of the pump, it was ordered from a parts store for a stock small block ford.

 

Sounds more like a little debris in the fuel line during the swap got into the needle and seats. Pull them, blow them out, reinstall them and adjust the floats.