/// I have several of this vintage. The usual problem is unregulated air. Second is a vibration caused electical problem. Run resistance measurements for all wires that go to ground. With the ignition on test for voltage drop at the same points. Put the negative wire on engine ground and use the positive to test all ground points. As you probe you may hear a relay click. I found one stripped mounting screw that way.

I found that the 73 B20 144 would not run with the battery out of the circuit. A defective negative battery cable was switching it in and out of the circuit causing shaking and vibration. If you have noise from the alternator the battery acts as a filter. Try putting your jumper cables in parallel with the negative battery cable. Last, try by passing the system wire. On my 73 it runs to a distribution block that is between the headlight and the battery. This is a cheap and quick method of being sure the problem is not in the basic charging circuit.