[Lynn Eades]

Back to the condensor.
Tony Generally speaking the condensor is matched to the coil. If the condenser is over capacity or under capacity it results in arcing at the points (contact set) and a transfer of material (platinum) to one or the other side, resulting in a short life to points. (btw, vented points are a better option if available)
If the condensor is u/s.(i.e. completely stuffed) it will often leave the points burned black.If it still goes, it won't for long. They should be a clean grey. (the only other possibility here is oil getting into the points)
You will probably notice that 6 volt condensors are bigger than 12 volt condensors. (this might be an age/ construction thing, all the same, its a good guide)
Sometimes you can buy one with the microfarad value stamped into the body, and in some cases they were available in different values for the same application.
Having converted to 12 volt you should change the condensor for one made for a 12 volt application. Any one you can get in the hole, and hook the wire up will do. All you need is the thing clamped or screwed to the distributor body, and the wire hooked up to the feed to the points. You will notice that the early jappers had them on the outside of the dizzy. Keep an eye on the points, and try a different condensor if they are tranfering metal faster than they should. The local auto electrician might be able to help regarding capacity up or down. If your coil came from a certain type of vehicle, a condensor of the same capacity, as that vehicles condensor, will do the trick.
I vaguely recall seeing in a wartime training pamphlet, teaching the drivers to make a tempory condensor using a bean can and paper. Maybe someone can recall.
Anyhoo you have it sorted. My guess, from your description, fuel.
Hopefully this has filled a few knowledge gaps, for some of the would be mecanical lurkers out there.