Discovered the reason for this peculiar condition after removing the starter/generator and opening the generator end.: Inside the Delco starter/generator is an insulated wire that runs from the armature current output brush to the contact arm on one of the starter brush holders that connects the contact arm to the "Armature" terminal on the rear of the unit. It runs between two of the field windings. Near the generator brush holder, the wire mentioned above passes across an uninsulated wire connecting two of the field coils. The insulation on the brush output wire had chafed against the field coil connecting wire, worn through, and created a short circuit that connected the armature output current to a portion the field coil circuit, allowing the fields to be energized directly, even if the ignition switch was open. If the engine was running at a speed above a very slow idle, there was enough current generated to keep the ignition system supplied with the switch open. When the idle was slowed to a minimum and the ignition switch was open, then the engine would stop.
The apparent remedy for this condition was to replace the wire with the failed insulation, which was done with modern insulated stranded THHN 12AWG wire. Not a simple task. The short circuit condition had apparently created some large amperage currents circulating through the internal connections of the armature, as there was evidence of solder being flung out of the commutator onto the inner surface of the housing end cover. It is all back together now, and starter works fine, but generator charging rate is too high, even with third brush at lowest setting.
A while back there was an article in Fork & Blade about installing a voltage regulator to the generator on K series Lincolns, which have a separate generator. It seems to me that this could be done for the Lincoln "L", as well. Just set the 3rd brush to max. output, and connect the regulator to the existing Armature, Battery, and Field connections. Has anyone done this? A 6 volt regulator for a post-war Buick, Cadillac, other large car or truck should work. Can someone send me a copy of that article? I can not find that issue of F&B.
Another approach to regulate charge rate is to put a large rheostat, 4 -5 ohm resistance in the field circuit between the ignition switch and the field terminal on the Delco starter/generator. I have a unit that was marketed especially for that purpose in the 20s-30s, to "Prevent Overcharging". it is a 5 ohm rheostat in a bakelite enclosure, intended to mount through the dashboard.
Any experience or opinions on this scheme??