If there are bits of busted brush floating around inside, that's not good news, although i wouldn't like to say it would be more likely to cause a consistent fault on one cylinder than a general malady. Given that a mag can send a spark a good quarter of an inch, a bit of spare conductor in there could cause a lot of arcing. The good news is that there are sparks on one side, apparently all the time - so the mag works.
However, there are other possibilities, and if you see no sign of the missing brush (or its spring?) inside, if I were you I'd have a good look at the contact breaker. Are the points gaps equal for each cylinder, or near enough? A big variation spells trouble. The bearing could be floppy (maybe the insulator washer behind it has disintegrated - they can do that), the hole in the fibre heel that the moving contact goes on is oval, etc, etc etc. Less likely: - If you've got manual ignition, the cam ring could be sitting a tad cockeyed in the end housing owing to wear in the latter. Even if you haven't got manual ignition, check the cam ring is a snug fit in the housing - if there's any wiggle room, that can be bad in terms of gap variation.
Also worth checking the earth bush is there and in good condition, although the thing can run without it, I'm told, and that should not be responsible for a one-cylinder problem.
I'm sure all will be revealed when you have the thing in your hand . . . whatever it is, it probably ain't that serious. Clean the slip ring carefully if you take the armature out (might be covered in black dust from remains of old brush?) - and watch out for the 'safety screws' - if you don't take them out of the mag body first you'll likely bust the slip ring trying to pull the armature out, which is a big pain. (If you leave the armature in, clean the slip ring with clean rag on the end of something blunt poked through the pick-up holes.) Note the shims, if any, between the end piece that holds the cam ring and the body, too . . . need them to be right to manage the endfloat of the armature. Make sure the points are clean and try to see whether they mate squarely when they are shut - they often aren't that good. Far easier to set their gap with the mag on the bench, too, as it's hard to get a spanner on the locknut and another on the fixed point when you're doing it with your head at a horrible angle and not much room, poor light etc. Fingers crossed. Groily
