Thanks, everyone. Here are answers to your questions:
Plug color is perfect. What kind of work has been done to the ignition advancer?
No work on the advancer - just removal and inspection. It seems to be advancing properly. I even checked it using a second timing light.
Sounds like the carbs are not clean and the mixture coming off idle goes lean. Jetting with that exhaust Might be an issue, but I do not think I have heard anyone mention they needed to play with the jets before, just the carb adjustments.
These carbs can still let the bike run in amazingly bad condition for the carbs.
Is the Uni oiled? It takes very very little oil for the UNI filter and it typically will run richer with one in use, not leaner.
What needle position and how many turns out are you on the air screws?
Carbs have had a thorough overhaul and appear clean, but maybe I missed something. I've had them out twice in the past week. I'll pull them again, as I know "carbs are clean" is something people often say before discovering it's not the case. Aside from the jets, are there any other passages/circuits I should suspect?
The filter is oiled. For the heck of it, I tried the old paper filter and it made no difference.
It was running fine before this project, so I didn't move the needles. Just can't remember what setting they're on now. I'll look at those as well.
On the air screws, I went for a ride and adjusted them 1/8 turn every little while. I did this from 3/4 to 3 turns. The sputtering is less severe as I back out the screws beyond ~2 turns, but it never goes away. And the backfiring on decel is always there. The above photo was taken at ~3 turns, which I figured is too far out.
If anything, I'd expect the fuel level to be a bit low barring anything out of spec with the ignition advancer. I doubt seriously that the exhaust with the stock airbox is causing any issues.
I'll check float levels and advancer again. Could low fuel level also cause backfiring on decel?
