‘85 truck. 318. Friend’s truck. Will not start after running a little bit. Ballast resistor is real hot. Resistor looks new. Can a door prong resistor be used in order to trouble shoot?

ballast is bypassed during start.
so if it was the ballast you would expect it to start then die when you let off the key.

It’s getting super hot. Quickly. Plenty of fuel. Spark to and from distributor. New MSD Blaster coil. After truck sits for a little while it starts.

Ballast resisters get very hot that's why they are ceramic bodied.

You're loosing an electrical connection somewhere after system heats up. Connection can re-establish itself after cool down.
Possibles:
1) connection between ballast terminal spade & coil wire inside it.
2) Dist. pickup coil/leads: I sure had this happen to me few years back.
Cut-out on hwy., & after some cool down time started back up.
Occurred to me 2 times before I fixed it.

That's it for now.

You lost me at "door prong resistor ".

Yea I can see that.
Four prong.

What is the resistance on the ballast? So you added new coil and ballast get real hot. That means excessive current and possibly coil over heating. Need to check the wiring, and make sure the resistor meets the coil spec.

Exactly . What ignition ecu, what coil and what resistor? They are designed to work together and you may have disrupted the combination with the Blaster coil. Different coils can have very different primary resistance. What works best on a CD ignition is probably very wrong on an Inductive ignition. If the total resistance of the coil and ballast resistor is too low the ECU life may suffer. If the ballast resistance is too low for the coil then coil life could suffer. If the total resistance of the coil and ballast is too high then spark strength will suffer. If everything works together they will have a very long life. Things tend to go sideways If you upset that ballance by changing a part to something inappropriate.

Not necessarily true. I had a ballast resistor go bad years ago on a '71 Charger & it would start & run just fine, then just cut-out at very inconsistent intervals.

OP, start it & then get out & wiggle the wires connecting to the BR - if it sparks & shuts off, then it's the BR.

I had one break!

Does it have spark when it will not start???

It does have spark when it will not start.

So if there is spark to the plugs you are either starved of fuel or flooded.

The point was that the ballast is not used during start. So if it won;t start it's probably not the ballast.

OP later does say that there is spark when it won't start so the issue probably isn't ignition related, unless the timing is way off. Jumped timing chain?

I think I got it a little closer tonight. I swapped out the coil with one with higher resistance. About where I would expect it around 1.5-2.0 ohms. Then the timing that was at about 5 degrees BTDC, I moved it to about 3-4 degrees ATDC. Then I noticed the starter motor was dragging just a tad after attempts to start the engine. I put a charger on the battery and it helped some. I at least could sort of repeat the failure and the ignition. It still has its moments where it does not want to start. So the next question I have is this- can the starter become weaker as it warms up? I can pour a cap full of fuel in the carb and it does not help the starting. I don’t think it is flooding because it is a 2V carb that I took off the shelf and it ran well when I put it on there. There is plenty of bad wiring under the hood. Some wiring has lost its insulation. So that stuff has to be cleaned up as well.
About the BR, should I be able to hold it in my hand while the truck is running without feeling like I’m going to get burned.
I think there are numerous issues working in according. It’s not there but I sa things tonight that encourages me that it will get there.
Once it starts it is slick, smooth and responsive.

No, ballast resistor is hot to the touch you wouldn't want to hold it. Like others have already said ballast resistor is bypassed during starting. If it's bad it will have no problem starting but will stall when you let go of they key. If it won't start at all its something else.

Right now my truck is hard starting and if you do get it running it dies pretty quickly. I got it down to a bad fuel pump.

Description of the issue and symptoms need to be more clear otherwise folks starting reading into your issue and then down the rabbit hole. I reread your first post. First, you start the truck and it is running. Does it shut off on its own or do you turn it off? Hard starting of a hot engine can be several issues including vapor lock of the fuel lines.

Yes you should start with a know good and fully charge battery. Good fuel with a know good fuel pump. Good starter. If it cranks over easy hot, and it does have spark (continuous when cranking) it most likely is a fuel problem when hot as long as you know it sparks not just on the coil wire, but at a plug wire too.

The overly hot ballast can still we an issue with a coil out of spec to the ballast.

A hot coil will mimic vaporlock. Shut it down after a drive and it won't start, let it cool and away you go again.

Timing should be about 18* btdc with the vacuum advance disconnected. To much advance will make it turn over very slowly.