I've just completed rebuilding the Bings, setting the valves, and checking the timing. I went out for a 10 minute ride before making idle adjustments and about 5 minutes into it the engine would not drop back to idle, just hung around 3,000 rpm. At 10 minutes I stopped, turned it off, restarted (at normal idle), and adjusted the mixture screw and idle screws for a 900 rpm idle. The left carb was almost a half turn higher on the idle screw than the left. Would that cause the initial hang-up? After the adjustments were complete I did city driving for a half hour without a repeat of the sustained high idle so I'm declaring it corrected. But I would like to know what the other possible causes are.
BTW, who ever suggested removing the air intake horns from the air box - good idea I think. I made too many changes to say it was the main contributor, but this old airhead is a whole different machine. Wish I did all this two years ago when I first got it.