‘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.

Install a $3.00 ballast resistor & see if the problem solved. If not, you have an extra BR.

When mine went bad in my 71 Hemi Charger, I was driving it & it just quit. I pulled over on the shoulder & it started right back up, so I continued driving. About another 2 miles it died again, repeated the same scenario until I got to my buddies' house & there it sat for 2 weeks. It was getting fuel, & getting spark when we tested - WTH? So I started replacing inexpensive parts: coil, cap & rotor, elect. ign. module (I had extras), etc. It would always start & run, but then would cut-out. I finally decided to make the trek back to my place about 25 miles away & it died a few times on the way, but I made it. I was about to take it to an auto electric place & tried fiddling with it one last evening & started it & then observed underhood while it was running - saw a spark at the BR & it promptly died. I started it back up as usual, got out & wiggled one of the wires to it & it sparked again & died. I had an extra (old used) BR in my stash & installed it - problem solved. My story............

You don't want your timing at 3-4 degrees ATDC.

Ok back up on the timing. Set to 18 degrees advanced. It runs cooler and starts better.
Someone earlier said something about the starter and battery. One or both are weak I think. If it does not want to start now I throw the charger on it at 50 amps it will start. Even at a15 amp sometimes it won’t start.
I think there has been a combination of things contributing and most likely the BR had nothing to do with it.
I have inherited some people’s hacked up work that has been going on for several owners probably. I think we’re making progress.

A bad ballast resistor will do that but it won't cause a no start issue so there's still something else going on.

Did you try a new battery? You should check both that and the alternator or just try a fully charged battery you know is good to try and see if that makes a difference. If it has a battery or alternator problem you'll be chasing problems down forever for no reason.

Had kind of a similar issue, hot sure if your car shuts down after running for a bit or nor. What happened with mine was changed my ECU box to a mint looking one because I can, LOL. So with this different mint looking ECU I was letting the car idle and warm up and it just stopped running?? so I went to restart it and it wouldn't start right away, after a while of sitting it restarted fine but quit again after a while, turned out that once the ECU box warmed up it just shut itself down till it cooled off, Not sure if this helps you out or not but if you have a known good ECU try that??

BTW. FYI. LOL. When it was given to me it was sick and weak. Today I did a one wheel burn out with it. Kind of a reward for messing with this thing. Threw this in to lighten the mood. This week I’ll dive deeper into the battery, alternator and starter.
I’m doing this for a good friend and trying to make the repairs without just throwing money at it. I’ll post updates.

Mine has the same cracked core, but the coils are OK and ~.7Ohms
I don't use it for ignition as I have HEI now.

What is the system V when off, and when running?
Does the Alt gauge move?

Once upon a time I was all excited to take a girl out on a date in a Jensen Interceptor, dam eng. wouldn't start when I was picking her up the BR went bad.

The gauge moves violently. I found an after market volt gauge under the seat. Apparently someone has been chasing the problem.

Had the alternator tested today and it is fine. The battery is good. The next step is the starter. However, I will say with some of the moves discussed here the starting problem is getting better. I think the MSD coil and the timing being too far advanced has the biggest effect.

All tuned up and running great. Replaced coil, wires, plugs and adjusted timing.
When it gets hot it still will not start but when it does not start the starter drags. The battery and alternator are good. So next step is too replace the starter.
Replaced valve seals and the inside of the engine was surprisingly clean.

After the starter is done I may have to post in the truck section because the 4x4 is not working. Plenty of fluid in the transfer case. Nothing obvious underneath. Right now don’t know if it is an electrical or vacuum problem.

Before changing the starter I would take a good look at the battery cables and other wires associated with the starting system. Make sure all connections are tight and clean. One time I had a problem with my starter turning slowly and it turned out the battery cable connections were corroded. I cleaned them real good and that solved the problem.

Another time I had a problem where the positive battery cable got too close to the exhaust and some of the insulation melted as well as the wire was exposed to too much heat therefore creating extra resistance.