Ol'55, I see you are an aircraft mechanic, so you are no slouch. Don't mean to talk down to you, just normal procedure for everyone.
1. Was the battery "fully charged" by using a battery charger or when you tested it, it was charged already?
2. How is the motor grounded?
3. How is the frame grounded?

Did you take off the battery cables and clean posts and cable clamp ID's yet?
After you clean them:
Check battery at rest, MM probes to pos and neg battery posts. Should be about 12V-12.6V. Start motor and at high idle, 1200 RPM, should read 13+V, and go up as RPM increases. Report.

Before continuing further, we MUST establish a baseline. Have to be SURE battery is good. Have to be SURE that charging system is working. The above simple checks will determine that. Keep in mind batteries can be charged up and look good, but they die under the first or second load. Happens ALL the time. So it would be good practice to have it load tested by a "professional" kid at the FLAPS. This would be your no. 1 priority next step.

Your answers and your test reports are all we have to be able to advise further tests. A little at a time.

MM= Multimeter

Right is right, even if everyone is against it, and wrong is wrong, even if everyone is for it. - William Penn