1978 F150. 6.6. 2WD. Auto C-6 Trans. 135,000 miles. Factory 2150 Carb. no modifications to drive tain. Fuel issue. My fuel flow is cycling. With the truck warmed up and at idle I can see the fuel filter fill up and see the fuel flow into the filter is strong. after about a minute the fuel flow starts to slow way down until the filter is almost empty. Just enough fuel to keep the engine running. After about another minute the fuel flow gets strong again and the filter fills almost completely up. And the cycle repeats. Fuel pump and fuel filter have been replaced. New fuel tank about a year ago and the fuel in the filter is nice and clean. Idle is good. At times I get a hesitation going down the road at about 45. I'm guessing this is from the fuel pressure cycling.
Fuel pump and fuel filter have been replaced.
There's not much to the fuel system on that thing. Test your fuel pressure. A lot of mechanical fuel pumps sold today are junk that fail quickly or don't work properly right out of the box.
Check fuel pressure - Battling Today's Mechanical Fuel Pumps:
Troubleshooting carbureted engine fuel systems:
Hesitation could be a carb issue, like a worn accelerator pump. If the carb has never been rebuilt or has not been in the last couple of decades it is way past due. When rebuilding a carburetor be sure to check for vacuum leaks around the throttle body shaft. If the carb body is worn where the shaft goes through it would need to be bushed.
You can find a repair manual at the link below which may help:
