Headlight problem

I have a 1977 Sportster, exhibiting the following problem. With the engine off, the headlight will come on, bright, with the key switch in the ignition on- light on position. With the engine running, and the key in the ignition on-light on position, the headlight is extremely dim at idle, and goes out as rpm is increased. The following checks have been performed:
Generator output - at idle - 8 - 9 volts, rising with rpm. Brushes and commutator cleaned.
The speedo light is connected to the same terminal on the switch, but is unaffected.
bosch style mechanical regulator in use. (new)
12v present at headlight plug for both low and high beam (running engine)

Rino

check your headligt ground

Found Problem (and More)

I found that the ground wire and oil pressure sending unit wire were actually going to the "A" terminal on the generator.........HMMMMM, how many other problems with the wiring on this critter!?!....so after unveiling the wire harness in it's entirety,....I find several "abnormalities"..but that comes with buying a '77 that somebody else "tried" to build. I have now aquired all the wire, and am "weaving" my own loom so to speak....as per the schematic in the HD tech pub. I will not settle for second rate work on "MAMA's" bike.

Thanks for response
Rino