You're using a generator, not an alternator, right? There's a possibility you've got an open leg or two in the armature wiring of the generator. Is the battery staying charged OK? If you can dismantle the generator and check out the armature, look for burned spots on the leading edge of some of the commutator bars, indicating a short in the windings attached to that bar. A "growler" test will confirm any armature problems, open circuits, shorts, or grounds. The trick is finding a shop that still has a growler, and knows how to use it!
Other problems might be dirty points in the voltage regulator, or an incorrect voltage adjustment.
Jerry
