Ignition problem, I assume?

I have a '74 850 with a persistent misfire, that changes sides at random. I've stripped and cleaned the carbs, including the pilot jet checked float heights, set the timing according to the Boyer instructions and fitted new plugs, leads and 5K ohm caps. If I clean the plugs it will run OK for a while, then it will stop firing on one side. If I pull the cap off and hold it a short distance from the plug, it fires again. Everything is as per the specs, and I've even tried a dual output coil (with a 4.5 ohm primary resistance). I don't want to lose what bit of hair I've got left, so could someone advise? Do the Boyers (red box Micro) go south without warning? My son's a bike mechanic (Japs, usually ) and even he's mystified.
Any help would be much appreciated.

What kind of plugs are you using, and what do they look like when you pull them out? Sounds like they are fouling.

NGK BP7ES and BR8ES. I've tried a few others, but they all end up the same. The running cylinder is a bit rich, but the other has no combustion marks at all.

Try Champion plugs in the 7 range.

N7YC or similar, depending on whether you need resistor type...

The running cylinder is a bit rich, but the other has no combustion marks at all

Click to expand...

At idle and low speed Commando plugs are usually sooty IME.
Is the dead plug wet or dry?
Is it misfiring at idle and when revved?
It does sound like the Boyer so have you checked the pickup plate connections? Assume you have.
Check the Boyer pickup coils resistance, they can breakdown occasionally. They are in series so if one is going down intermitently neither cylinder will fire consistently.
Another possibility is your bar cutout switch and that needs to be eliminated as a source of any electrical probs. Try bypassing it and running power direct from batt to Boyer supply.
Check the engine earth as well, not sure where it is on Mk2 but it may be fractured and only making temp contact. Check battery end as well
Some of these ideas are straw clutching but elimination of any possibles will help you home in on the source.
All these ideas are valid as long as the battery is fully charged which I assume is the case or you wouldn't get it started!
And check the carbs again, you may have cleaned them but any tank dirt can quickly block those little idle jets. A dry plug is usually a clue (but not always)
Good luck!!

Thanks for all the responses, guys, now I think I have the solution. During daylight hours the bike runs fine, but On an evening run after some time with the lights on, the misfire returns. Visiting Boyer's website, and FAQ, I decided to check the battery. Charging was OK, but the battery was not holding the charge for any length of time. Replaced with a new item, problem disappears.
Once again, many thanks!