I have a problem with my truck I've been tracking for a while. I have a 1985 F-150 XLT Lariat MPFI 302. Truck starts great when weather conditions are normal, dry. Let a dew, light shower or drizzle come through and my 2 second pressure up turns into a long low continuous buzzing and a NO start. However, spray a little carb cleaner in a vacuum line and it starts. It has low power but runs. After 10 minutes of driving, the truck will shut down momentarily and pick back up and be running great then. I have checked grounds, replaced TFI module, magnetic pick up and ignition switch. I'm at a loss guys, please help.

 

look on the bottom of the fuel pump relay ( the green one)
It is probably corroded , more than likely the contacts are almost corroded together
NAPA makes a harness repair kit that replaces this connection. Add a new relay to this part, and you should be back in business