I've had intermittent terrible misfire on my Pinto. Finally traced to a knackered carbon brush in the centre of the distributor cap. Fitted a new cap and rotor arm from Burton which fixed it entirely for a month or two. Last trip out, terrible misfire again that cured itself on the way home. Just looked at the dizzie cap again and the carbon brush has disappeared. Any ideas why it should be eating the carbon?
Both caps were Intermotor side entry and last time I fitted a heavy duty rotor arm.

if the carbon look,s like it was burnt away, i would suggest the spark is either jumping a gap instead of the carbon rubbing on the rotor or there is a big resistance somewhere down the line, either leads or plugs

I spoke to Burton's and was assured that this is not a common problem. He suggested, and I think he's right, that I damaged the carbon fitting the cap. It's a side entry cap that sits tight under the twin carb inlet manifold and is quite tricky to fit. So I've ordered a new one and will be ultra careful about fitting it

Check for end float on your distributor shaft, if its excessive it could be ‘hammering’ the contact.

I had exactly the same thing occur on my side exit dizzy cap fitted to my crossflow - the carbon contact had completely burnt away. I never knew the cause either, but I did end up replacing the spark plug leads and coil due to a poor spark soon after replacing the cap. No problems since.

Ive seen xflow caps with just a carbon button for the central to coil lead, and had problems with them, as I belive the spark jumps across to the rotor arm, I now always use a cap, that has the carbon central node that is sprung

Also I believe bosch rotor arms have a springy metal top to them that is designed for the button dizzy caps, although im not 100%

steve