I think at this point we've reached the "take the covers off the generator and have a poke around with a meter" stage.
Is there continuity (from the ammeter - the lead that goes into the generator) to the negative (chassis) connection? That should show the resistance of the generator rotor windings (maybe with the field windings in parallel). If it's very high I'd suspect one of the brushes is not making contact with the commutator, or else there's something (a layer of dirt) in the way.
I'll see about downloading the manual later and see what I can come up with to check.
Best regards,
Chris.