She always looks better in the twilight...
Where did that bike come from? I have only seen those carbs once before, and it wasn't on a North American bike.
My guess, based on the type of idle-mix screw you have, would be a setting range for those of between 3/4 and 1-1/2 turns, richer as the screw goes out.
But, don't overlook these other possibilities:
The sparkplug heatrange should be 7, not 8. And the X22ES-U plugs (heatrange 7) tend to work a little better overall with those early-style mixers. While this won't necessarily affect the idle, it will help them stay cleaner: also use Regular grade gasoline instead of Premium today, as our modern Regular grades burn closer to the rate of 1970s era Premiums. If there is some unburnt fuel in the intake passages (which is a 100% possibility in the 500/550 intake design) it will help if it has a lower boiling temperature - like Regular grade does.
Try retarding the spark timing (under a timing light, for accuracy) about 2 degrees to see if it improves things a little bit. If so, then the spark advancer springs are sacked from heat-annealing (softening) and they are not retracting the weights as soon as they should during deceleration. This is EXTREMELY common on the 500/550 engines, because the springs were marginally too soft in the 1970s. I recommend cutting off 1/2 turn from both springs as a start: most of them I fix take 1 full turn off both springs in the end to make them idle down well when hot, and the slightly slower advance reduces the flat spot that happens just as you are fully releasing the clutch, too. This also helps match fuel burn rate and spark advance angle.
