Ok, so I went for a good long hour-plus ride. Missed all your messages in the meantime; they're much appreciated.
Based on the unassailable fact that the idle was overall too rich, I did some adjusting in a parking lot once it was warmed up. (Part of the reason it was overly rich was because my previous fueling problems had left it running lean much of the time, so when I set the idle mix, it wasn't against the proper fuel level in the carb...)
Both air screws are now backed out 1 and 1/2 turns...the jets, however, are 25s vice the spec'd 20s. (again, part of the overall lean mix from before...can swap the 20s back in now...)
It ran much better. I couldn't get it to idle properly under 1400 RPM because it was so rich; now it idled fine around 1100.
video-1588536323 by
Mick Doul, on Flickr
I did not do a cylinder-by-cylinder tuning out on the road, but was pleased that this seemed to bring things back into normal
That said, I had other issues. At one point, RPMs started to raise on their own, and I turned on the reserve petcock, which calmed it down. But I had filled up yesterday and shouldn't have needed to be on reserve...plus I was able to turn it off later and it was fine again. So there's still something restricting fueling here. (Or it was another cable issue, but I checked that and it seemed OK...)
Got back and the plugs didn't match, though neither was dirty compared to before. The RH cylinder was a light tan insulator and no soot to speak of, save a little on the rim of the plug which wiped right away.
The LH was a much darker insulator with a little soot on it and a little more on the rim, just like the RH had. I'm fine attributing this to a need to tune each mix individually now. Sound right? I know the carbs are going to be a little off of each other.
It didn't want to start again when I put the plugs back in, either. I had wire-brushed them a little but not given a detailed clean. Both showed massive sparks from their respective leads. (Somewhat to Knut's point...) I'm not suspecting the magneto at all. It's shocked me once or twice and compared to my Enfield with a boyer and coil, it feels like a defibrillator...
Thanks again for your help and patience...I'm learning gradually.