Probably your rectifier, Gunner. Batteries, back then, were pretty basic chem-tech; nowadays, they can absorb an extra volt or two. If you've not done so already, upgrade your rec to a silicon- or Schottky-diode type. Also, as Vince wrote, check your rotor isn't stamped 'CB72 (L)' -- that's a low-flux (magnetism) type.
I think they're all stamped CB72.
LED Tail Light Conversion
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