Swapped injectors for 2 and 3, and I'll be darned... it behaved like I expected. 3 warmed up while 2 stayed lower. I took out the injector that was originally in 3, sprayed some Berryman B12 through, used some compressed air, and replaced it in cylinder 2. It warmed up more after this, but I still wasn't sure if it was firing all the time. Poured a bottle of B12 in the fuel tank, and took it out for a 40 minute drive to see if it would clear out. The car seems to backfire less now, and it's not quite as high up in the rpm range when it happens. Also feels like I'm running on 5 1/2 cylinders now (a little more power). I think I'll take the injector out again once the car cools down, and use our ultrasonic cleaner at home to try and de-gunk it as much as possible.

What kind of solution should I use in the cleaner? Just water?

Further cleaning didn't seem to help much... so then I swapped the lines from the fuel distributor to injectors 2 and 3 while keeping the injectors in place. Behaved just as when I had done before, when swapping the injectors AND lines. I believe we isolated the main problem this way. Though there was no obvious blockage in the line, I ran some cleaner and compressed air through the #3 line, hooked it back up, and the car ran better with minimal backfiring. All cylinders were getting up to temperature and sizzled when I sprayed them with Windex. The other FD to injector lines may benefit from this service, but I'll leave well enough alone for now. I think the rest is down to ignition timing, which I'll try to dial in again tomorrow.

Phew. I guess this is my first-hand experience of how touchy CIS can be. But hopefully this could be my answer!

Great job on troubleshooting! CIS can be a pain. New injectors are pretty cheap. Cost is about the same if you send them off to be cleaned. Check the injector O rings for leaks. Keep up the great work!