78 cutting out when warm
 

Usually starts after about 20 minutes of driving. The fuel pump (Edelbrock Victor Jr), plugs, and wires are new. Distributor cap was replaced late last year. It’ll start missing first, and quickly decline until it dies. Always starts back up, but continues cutting out. My initial thought is the distributor itself. It’s definitely getting fuel. Any other suggestions on what to check?

sounds like your coil

Good call, I’ll try that first. Thank you.

It definitely sounds like a coil type problem. Keep in mind other things that have the same symptoms. The ign. Module, (inside the distributor) and the pick up winding, (also in the distributor).
None the less. The coil is easy to change and highly likely.
Classic coil failure symptoms.

PhenyxC3,

When you remove the coil for inspection, note the little brass spring under it.
Note that it also protrudes under the dizzy cap also.

The "Bushings" (spring) come in low resistance and high resistance. Most distributor caps come with high resistance bushings whether GM or MSD, etc.
The High Resistant Bushing run hotter.

You can switch to a low resistance by checking out Summit, under the IGN category. The bushing often comes with a new rubber insulator washer too.

When you replaced your distributor cap, did that include the coil? Do you have your old one to swap?

Pick up an HEI module, too, if you order parts, and some heat sink compound. Even if that's not the problem now, it's an easy spare to carry in your car.

My shop did that for me, going to check the paperwork.

It does sound like an Ign. module problem but don't rule out the coil either. Usually heat related problems associated with Ignition systems stem from expansion and contraction of the circuitry contained within the module. The repeated expansion and contraction creates stress risers within the circuits and eventually breaks the copper pathways within the module. So when heated the pathways expand and open the circuit and when cooled the pathways contract and close the circuit. Coils can behave the same way but to a lesser degree since their pathways (wire) are more robust than a module.

Most aftermarket HEI Ignition modules are junk, period. Get a Delco Ign. module. I would even trust a used Delco Ign. module over a new aftermarket one. That's how bad they are.

https://cimg7.ibsrv.net/gimg/www.cor...47f17c106.jpeg
Is this the factory distributor for a 78?

Ah, no. Aftermarket. Flamethrower.
But same concept, same innards.

On the top of that unit is a dust cover for the coil. Three little screws and the cover is off.
Now note one corner of the coil, which looks like a big magnet will have a black GRD wire going under one of the four coil mounting screws.
Make sure the coil, screw, wire, washer are all shiny. No paint or corrosion.

You will also be able inspect all the wire connections leading into the cap now, with the dust cover off. Are they tight?

Also note that your cyl #5 and #7 wires are crossed. Always keep those two plug wires well separated, from the dizzy cap ALL the way to the plugs.
Due to the firing order of Chevy V8s those two interfere with each other known as "Inductive Crossfire". Use your wire looms to separate those two.

Remove the four coil screws. Coil lifts right out. There you will see the bushing (spring) I spoke of in post 5.

All this is simple maintenance. You don't need a repair shop.

But you do need to separate plug wires #5 & #7.

Thank you, I really appreciate the help.

Actually, in that photo you put up about all we can see is the coil cover and a bit of the distributor cap. The distributor is under all that. And we really can't see it.
You could very well have a factory distributor under that cap and coil cover!
Or Not! Try to get a photo, standing on the passengers side of the car. Get down low and show us the side of this distributor from the clamp on the intake up.
Then maybe we can give you an honest answer as to whether or not you have a original distributor.

https://cimg9.ibsrv.net/gimg/www.cor...e4cbb7b9f.jpeg
https://cimg1.ibsrv.net/gimg/www.cor...f8c250635.jpeg
https://cimg3.ibsrv.net/gimg/www.cor...6cf4e6f21.jpeg
This is what I’ve got. Going to replace the coil and module while I’ve got it apart.

The coil is usually sold with the cap. Do you have a "before" picture?

Do you know what your timing is set to? It looks like your (adjustable) vacuum advance is not connected. Perhaps you don't have enough advance. Noticed any overheating, or run-on/dieseling at shutdown?

Grab a can of Dust Off. When the car dies, spray your electronic components one at a time and try to start the car. If the problem is heat related, it will go away, at least temporarily. Enough time to sort out the problem. Jerry

Well, some bubba sure likes to play with vacuum hoses. Good grief. More splices and poor connectors than needed.
And the second most important vacuum port, Intake manifold port, is capped off. ???????

And like Bikespace said, your distributor Vac canister appears nonfunctioning.

As I stated earlier, the black GRD wire under the coil cover: Wire brush that coil frame connection. Its an important GRD.

If your new module does not come with a tube of Heat Sink Compound, do not install until you find some.

Separate plug wires #5 & #7. Do not let them touch each other.

https://cimg9.ibsrv.net/gimg/www.cor...a165aec50.jpeg
Let me pan out. This might explain the funky vacuum lines. All vacuum has to come from the manifold.

https://cimg0.ibsrv.net/gimg/www.cor...443047eef.jpeg

You really should have started with this picture, in your first post. :jester

Now I see a vac line off the Intake from that angle.
Bad-ass charger.

Took some creative ingenuity on the upper rad hose too.