Why are my distributor cap terminals burnt?

I agree that phasing CAN be a problem but don't see it here......looks to me like the "most" of the arc is right in the middle area of the contacts.


Also just for clarity, it is not mechanical advance which causes phasing problems, but vacuum advance. The rotor and the trigger device are locked together

yea looking back at his photo you’re correct. The arcing is fairly well centered on the pins in the cap.