1. Raise the 3rd brush
2. Loose the four screws that hold the brush rigging
3. Apply 6 volts to the output terminal
4. If the armature shaft turns, rotate the brush rigging until the shaft stops rotating.
Seems easy enough, but sometimes the armature shaft will not rotate at any position of the brush rigging. Then I remembered how we used to set the brushes on DC machines at the power plant. AC current was passed through the field and AC voltage was measured at the output terminals. If the output voltage was more than 1/2 volt, the brushes had to be adjusted. I thought I'd try a variant of this on the Model T generator. Since this is basically transformer action, the source AC could be applied on the armature and the results measured at the field. Since the Model T armature can withstand more than 10 amperes, I used a 500 watt lamp to limit the current from the 120 volt wall outlet. Then with the 3rd brush raised, I measured the AC voltage at the field. Rotating the brush rigging, I could get 4 or 5 volts at the field and by rotating the brush rigging the resultant field voltage could be reduced to about 0.25 volts. I deemed this a success and will use this method in the future. Here is the circuit for my setup.
Mike