Confounded by even the simplest electrical problems

Adjusting your third brush generator is as easy as falling off a log. Open the band cover on the generator. The third brush should be redily visible. It is the one that has a slot headed machine screw sticking out of the end plate next to it. Loosen the screw a half turn and slide the third brush either up or down to change the charging rate. If you have the engine running and use your finger (carefully) to move the brush holder around you can have someone sitting in the car and they can tell you exactly how much you have changed the rate. Otherwise move the brush holder, tighten the screw and start the engine to check the charging rate. You might want to consider installing a 6 volt alternator??

Happy hobbying.