I have a 1984 ford f-250 460 4x4. and i am getting minimal gas in the carburator causing the truck to only run for 10-15 seconds, i give it gas when it starts to die but it still dies. i ran an alternate electric fuel pump outside the truck from the tank switch over(i have two tanks), and the truck runs perfect. I have changed the fuel filter , rebuilt the carburator(and adjusted the floats), changed the fuel return valve and fuel relay, I also checked the roll over shut off valve and reset it. I also checked all the fuses. Any ideas on what could be wrong? Please help i am out of ideas.