Carbon tracking through rotor. More HEI woes.

Is the spark going through the plastic to the dist shaft, or along the outside.

Tru HEI rotors have a lot of material between the dist shaft, and the contacts.

You could try an experiment, and cut a disc out of a plastic milk carton (or the like) and put it under a new rotor in such a way it won't interfere or come loose and see if doesn't help or cure the problem