Motorcycle Repair: 1978 cb750 kz (dohc), battery tank, bench test

Question
Hi Chris.  I've just helped resurrect/renovate a '78 cb750 that'd been stood 2 or 3 years - all the usual stuff like head overhaul, brakes, battery, tank & fuel lines, THOROUGH strip/clean etc of the carb, new plugs, good sparks etc. Now runs well most of the time but cyl.4 (& sometimes no.2 )don't fire cleanly and run cold until we put clean dry plugs in. Done the usual HT lead swapovers and doesn't make any difference. Carbs are set up ok and don't flood. Any clues/ suggestions/advice/ what to try next please?  Thanks.  

Answer
Hi Colin.

 Have you tried replacing the coil with a known good one?
 Have you done a bench test on the offending coil as per the repair manual's proceedures?

 It sounds like that is the problem here.  Everything else seems to be just fine.

Let me know what you find.
Good luck and ride safe.
FALCON