Or check if both plugs and caps are resistor type. Resistor caps OR resistor caps is ok, both are not.
But still, if the hi voltage is too weak to fire plugs at the correct gap, something is off.
- Poor battery voltage, which seems to have been ruled out.
- Voltage at coil + is too low (again, poor connections in harness, ignition switch, kill switch. On my 77 this was bad enough to not make any spark at all. Fixed it by doing the "relay mod", that the OP rejected)
- Too much resistance between coil - and ground (bad connectors, bad or dirty points)
- Insufficient dwell time to reach saturation in coils ( worn points cam, slop in advance shaft, low quality points)
- Bad capacitor/ condenser
- Bad coils
Bad grounding of points plate, capacitor/condenser, or the engine itself.
UPDATE:
Voltage at battery terminals with ignition OFF: 13.4V.
Voltage at battery terminals with ignition ON: 13.2V.
Voltage at battery terminals with engine idling at ~2,500rpm: 14.1V~14.4V (it increased as the engine warmed, although the rpm's remained the same).
Voltage at battery terminals with engine idling at ~1,100rpm: 13.6V.
Unfortunately, today, we're back to the right cylinder
backfiring at ~1,100rpm idle (but OKAY with choke ON and idling at ~2,500rpm). Pulling the spark plug lead on the right cylinder has NO effect on idle. Pulling left spark plug lead stalls the engine. Less than two minutes later, the problem shifts to the other side, but without the backfiring.
Once again, I pulled the points cover and, as best as I could tell (overcast daylight sky) both points were sparking. But it's difficult, at best, to tell how strong the sparking is whilst observing things in the middle of a busy public parking lot (again, the bike's stored in a public storage unit and tests/repairs are prohibited on the premises, so I have to work quick and "under cover").
Anyway, at that point—cold and VERY frustrated (this's been going on for nearly two years)—I packed it in for the day. I apologize for being unable to perform some of the other requested tests, but I'm now repeating alot of the same tests/checks for the third and fourth time. Ugh (frustrated).