Hi, Just bought a 1978 ford F150 Custom with a 351M motor. It has sat for a year. Cleaned out the gas tank, new gas and fuel filters, new cap and rotor. Truck starts up good and runs about 10 minutes, then it dies like the key was turned off. I know the gas isn't the problem. It loses spark when it warms up. I need to determine what is wrong. When warmed up, it only sparks once as the key is turned and not while cranking. I think it is the brain box, pickup in the distributor, or the coil? Any way of checking any of these? Best guesses on what to replace first? Also, does the distributor have to come out to change the pickup? Thanks! John

 

the distributor can remain in place. after removing the cap and rotor. the armature is located with a roll pin. the armature will slide off the distributor shaft, with either a lifting action or very delicate pry action with a couple of screw drivers. the next part that needs to be removed will be the armature stop ring and the wire retaining clip. you will also need to remove the retaining clip that retains the vacuum advance arm. the stator assembly lifts out, but be sure to detach the system ground screw located where the leads exit the distributor housing. welcome to fte and good luck with the distributor.

 

I would say the brain box. It dies once it warms up. You should be able to change the pickup with the dist in there, i would take the brain box in to your local parts store and have them test it.

 

I took the brain box to autozone. They ran a check on it 5 or 6 times and it passed each time. I am still not convinced that it is good though as the test only takes a few seconds and the truck runs good for ten minutes. Thanks for the replies! John

 

If it works when cold, thats when they are testing it. It still can be the ignition module. i keep a new one on hand and test it when i have a problem, might want ot buy one, because even if it is not the problem, it will be sometime down the road.

 

I changed the brain box. No Improvement. I will try the pick up located in the distributor next, unless someone has a better idea. I've got to get this old Ford running good again! John

 

Chances are good that a new pick-up coil will fix it. Also check the wiring to the pick-up coil. It tends to break because of the flexing caused by the vacuum advance. Make sure the (black) ground wire attachment is clean and tight.