That configuration should be producing around 19 volts at speed.
I don't know why it's working as a starter, and not as a generator.
Looking at the diagram, you can see it's nothing more than armature coils, and field coils.
http://www.buggiesgonewild.com/attac...n-retrofit.jpg
I guess if there was an internal short where the df wire connects to the field coil, it could kill the generator part.