Gaining access to that brush required removing the generator — a closer look revealed dodgy insulation on the coil wires and failing brush plate insulators.
I replaced the brush plate insulator strips with the new fibreglass(?) ones, re-insulated the field coil wires, and replaced the terminal wire. So far, so good.
I then locked the brush plate in the neutral position (12vt+ to the output terminal, —ve to the case, 3rd brush lifted) where it would not motor in either direction (actually it really only motored in one direction and barely tried to motor at the other end of brush plate travel). After dropping the 3rd brush onto the armature, the generator motored nicely on 6-volts and drew about 4-5 amps. Again: so far, so good.
Reinstalled the generator on the engine, fired up the engine and... nothing. Zero output showing on the ammeter throughout the full travel of the 3rd brush. So far, so bad.
I experimentally bypassed the Fun Projects regulator with a jumper wire but once again, zero output. I connected a 6-volt bulb between the generator terminal and the frame but again, nothing.
Although the MTFCA Electrical System book has been generally helpful, from what I've read this darn thing should be working just fine.
I'm at a loss as to where to go from here. Could the problem possibly relate to the neutral setting in that I couldn't achieve active motoring at each end of the brush plate travel? If not that then what? Inquiring mind wants to know...