Hei conversion Intermittent start?

The cap is most likely not the issue. The HEI voltage will jump across it.

Check 3 things and let us know:

1. Unplug the wire from the distributor to the HEI module, and connect your multi-meter to the two wire, in DC millivolt mode (200 mV or less). Crank the engine, and see if the multimeter readout changes. You should get abounce of between -60 and 60 milliVolts. This will confirm you have a good tach signal to the HEI.

2. Confirm you have voltage to the HEI when the switch is in the "run" position, by checking the voltage directly from the positive HEI blade, without unplugging anything, and the ground. This voltage should be identical (+-0.1 volts) to the battery voltage across the battery posts.

3. Check the voltage to the HEI when the engine is cranking, voltage directly from the positive HEI blade, without unplugging anything, and the ground. This should exceed 11 volts, and be identical (+-0.1 volts) to the battery voltage across the battery posts while cranking.

If all 3 of these check out, then we have effectively eliminated the wiring and voltage as problems.

Triple check the wiring polarity from the distributor to the HEI. Even if the wiring is correct, swap the wiring polarity (switch the two wires top to bottom). Try firing it again, just to see.

What is left?
Coil
Module
Coil wire to distributor