I have an 82 F100 with an 300. It has over 200,000 on the engine. In 08 removed most of the emissions head ache and put on a Clifford 6=8 intake, the split EFI exhaust manifold, dual standard tube exhaust, and a Holley 390 CFM carb. I installed a electric inline fuel pump that is putting out about 6-8 PSI. The was bought fresh from the store with no alterations. I have the 3 w OD on the floor manual transmission that was rebuilt in 08 as well. The truck ran quick, (horrible gas mileage, but I knew that was going to happen) quick accelaration and fun to drive. However about 2 months ago I started having fuel issues. So I installed a new electric fuel pump, fuel new fuel sending units in each of the tanks, changed all the inline fuel filters. Now the truck is still has quick acceleration in 1st and 2nd, but when shift into 3rd its like it hits a wall. Its like the truck just stops accelerating at about 45-50 MPH. I do have 33 inch tires on it but I have for the last 4 years and it was fine before. I gave the truck a tune up about 2 years ago, with new plugs, plug wires, but did not change the coil or the distributer. I love working on the truck but Im not sure what might cause this. Could this be a carb issue and it just needs a cleaning or a timing / electrical issue?

 

Check the timing, electrical and ignition system first. Depending on how much you drive the ignition system might be overdue for a tune-up. Once that is all up to snuff start on the carb and fuel delivery system. If the electric pump is adjustable, back off on the pressure a bit. 6-8# might be a bit too much.