I own a 1978 F150 4x4 that has developed a very strange loss of spark. Here is what happens..

I start the truck in the morning and it fires up fine, I drive it for approx 20 minutes or more & sometimes less then shut it off. When I return and go to start the truck, the motor turns over fine but acts like there is NO spark (almost wore out my battery trying to start it). I checked to make sure it's getting gas and it is (gas is squirting into the carb from the squirters). After the truck sit's for about 15-20 minutes, I'll try to start it, after a few tries then it starts up just fine as if nothing was wrong. I changed the ignition control module and the ignition switch and the problem still happens. the motor runs cool and the problems happens no matter the air temp outside. It's done it in 58* weather and in 85* weather.
it's as if once the motor is fully warm and running for a short while, it needs to cool down for 20 minutes or so before it will start up again

I had a similar problem about 3 years ago when the truck would just die "while driving". It turned out to be that piece inside the distributor (forgot the part name). The mechanic said when that part got hot, it kills the spark. Could that piece go bad again so soon? It was replaced 3 years ago.
I called him and told him what is happening now. He said because the problem is intermitent that all he could do is hook up a fuel guage and his engine analasys machine and wait for the problem to happen before he would be able to trace the problem and that could be many days and still not find the problem unless it acts up, so I could end up paying for many many hours of labor and still have the problem.

The motor is brand new (2000 miles) as is the fuel pump, 4 core radiator, carb, ignition switch, ignition module. The truck has been working perfectly ever since the new motor was installed last year, and now this gremlin pops up and I can't find the problem because it happens intermitently. I never know when it's going to happen. Obviously by letting the truck sit for 15 minutes or so is causing something to happen since then the motor fires right up and runs perfect till I shut it off, then after sitting for a short time, 5-10 min,it acts up. It's not flooding or vapor locking.

I hate to take the truck to a shop only to have the problem not occur and waste $$$ and not find the problem.

Does anyone know what might cause this kind of intermitent problem?. It's very irritating to drive some place, shut the truck off for a few minutes, then go to start it and hear just the motor turning over, but not starting.

PLEASE HELP!

 

Ignition Coils don't last forever either.

Try looking at it at dusk, pull the coil wire out of the distributor and touch it to ground, Note the strength of the spark when the engine is cranked.

 

i think if i remember right the pickup wire in the distributor is bad or cracked.. i had a friend to have that problem.

 

He's right about it being hard to diagnose an intermitent problem. What I would do is arm myself with a few items and be prepared to check it when it quits. When it dies, I would go out and do the screwdriver in the sparkplug wire trick, just to be certain you don't have spark. Then I would take a meter or a test light, and hook one end to the positive of the coil, and the other to ground, to make sure you are getting voltage to the coil. If you are getting voltage, I would make sure you are getting voltage to the red wire on the module. If all that checks out, then I would look at the wiring between the module and the dist, and yes, go ahead and change the pickup in the dist again.