One1 wrote: . . . Still very slow to return to idle when revved any amount above idle so it's not right yet. I feel like it's going in the right direction. Idle is smooth, about 1k right now. It used to come back down to idle and lope and die. Now it comes back and stays at strong smooth idle, even if it takes 4 second to get there. That's progress . . . .
Is it for certain that all the carb slides seat fully down immediately upon closing the throttle? (No frayed cable. Cable is lubed. No crud inside the twist grip. Adequate slack in throttle cable. No binding at the carb end. No issue with the carb return spring.)
With such mechanical issues ruled out, some minute cracking may have developed in the carb holders, especially if they are old. Attempted sealing of such cracks is usually just temporary. Brand new carb holders are the lasting fix.
With engine idling, may attempt to locate an air leak by spraying WD40 or carb cleaner onto the holders, which sometimes allows the spray to be ingested and affect the rpm, and perhaps also emit smoke from the exhaust.
Good Fortune!