Technical Doesn't start when warm

NAILED IT!
It was the 50 year old wire that lead from the ignition switch, to the solenoid. Don't ask me how it only worked when it was cold and not after it ran for 20 minutes or more...

A GOOD multi-meter is gonna be your best friend in times like this, so you can check for a bad wire with a VOLTAGE DROP TEST. Connect the m-m between two points (switch and solenoid in this case) with the circuit on. If your reading is around .5 volts or higher, the wire has excessive resistance, can't carry the juce the circuit needs.
I bought an INNOVA meter at Walmart for around 25-bucks, just for my travel tool bag a couple years back. The key feature I like is the AUDIO CONTUNITY TEST (aka 'buzz-box') for checking grounds and connections, tiny bulbs, etc. I don't use it often, but the more I do, the more I like it!

HEI ignitions are very easy to troubleshoot. First, make a spark-tester from a junk spark plug, 10-ga. wire (12-15" long) a 'gator clip and mini hose clamp. Open the gap on the plug to .80" or so, connect the wire to the steel housing of the plug with the hose clamp. Use a large 'gator clip to connect to a good, clean ground. The long lead lets me locate the plug so I can see the damn spark! Test at the coil AND a random plug wire.
Testing for a bad module is as simple as connecting a test light between ground and the (-) side of the coil. With the key ON, the light should glow BRIGHT. Crank the engine, the light should cycle between bright and dim for a good module. If the light remains BRIGHT and doesn't cycle, module is BAD.

Electrial **** can and will drive ya NUTS, trying to pin down the problem!!! HOPE this makes it a little easier for the next time! - Tim

 

Thanks! Same exact issue with my 70 350 in my 55. I will be checking it out today!