Well, in the end that's what we did. The car is running gents, thanks for all your advice.

To tie this up and help the next poor bugga with this problem out, here's a summary of what went down:

The car was a daily driver when purchased, and was due for some love. I took it off the road and started the process of restoring it. Front bench off to the trimmers for repair, new carpets fitted. Heater box out, DMT foam kit rebuild completed. Whilst the cabin was stripped, I took the opportunity to remove the whole dash frame and check the wiring....usual black alternator wire damage at the bulkhead connector noted, had been bypassed with a feed direct to the starter relay stud. The ignition switch connector had been removed at some point and each wire was connected with bullet splices...not ideal.

Whilst waiting for the seat to come back, and with rest of the cabin under control, I did some brake work, fitted new Rallye's, and changed out the the old ballast resistor. They're cheap and they often fail.

Heater box back in, dash installed, seat back in....turn the key. Crank but no start. Check the bulkhead connectors, all in. Ignition switch wiring....still dodgy, but all connected correctly. Ballast, plugged in. New ECU fitted (NOS MP Orange Box). Whilst it worked when I parked the car for restoration, the original ECU had clearly overheated at some point in the past and the potting compound had melted and leaked down the firewall:






Hmmm, not an ECU issue. Had them on hand, so fitted new cap, wires and plugs...no joy. ECU earth checked, seems good. Try a known good coil, no joy. Got this NOS MP distributor on hand, fitted...still no start. This is getting weird. Bit more wire fiddling and head scratching, yay, car runs...for twenty minutes. That's when we discovered the "one spark" situation as described earlier. Car now seemed to start semi-regularly, but only when it was cold. It would then only run for a short period before dying.

It was like the key was being turned off, so maybe the ignition switch was dodgy. New switch installed, and an old Barracuda donated it's connector so the bullet splices were now history. Turned out not to be the ignition switch.

The engine harness had been patched a dozen times over the years and was going to be changed at some point, now seems a good time!! New harness purchased from Classic Ind and installed, problem remained. The old harness fell apart in my hands, so I was glad to be rid of it anyway.

Thinking cap on. What had changed since the car last ran?? Ballast resistor, and bulkhead connectors disconnected when the dash assembly came out. So....tightened the female spade terminals for a more positive contact, no joy. That's when I discovered the ballast resistor was manufactured incorrectly.

What this meant was this: I had .5 ohm in the auxiliary circuit to the the ECU. This circuit should have 5 ohms to protect the ECU, but since I now had a four pin Orange Box in the car now, no harm no foul as that circuit is now redundant....the .5 ohm was connected to nothing.

The other side of the ballast put 5 ohms into the coil primary circuit. This would have reduced the primary voltage and therefore reduced the coils output to the distributor and the plugs. May have contributed to the hard starting but clearly not the cause of the cutout whilst running.

This is when the good denizens of Moparts came in

Having read this thread and spending time this past week boning up on the intricacies of the "new for '72" Chrysler Electronic Ignition, I shaped up to the car yesterday ready to win this bout! The car started from cold again, and cut out eight minutes later. Multimeter out, and measured the new MP distributors pick-up coil.....1.2 Meg Ohm. One million, one hundred and ninety nine thousand, four hundred more ohms than specified in the factory manual!!

Original distributor back in, car ran like Carl Lewis in 1984.

The autopsy:

The dodgy new Standard Parts ballast resistor from Rock Auto likely caused hard starting, during which time a faulty new MP distributor was installed. After cooling down having been removed and put on the bench, the new MP distributor pick-up coil measured it's nominal 600 ohms. I had an intermittent pick-up coil fault in a brand new distributor.

Thanks once again to all who chimed in. I lost a few days changing bits, but they were mostly due for replacement anyway. Along the way I became a Mopar wiring expert and had the interesting experience of dreaming about running small block engines!

Cheers,

Mr G.