Trying to determine cure for an intermitent fast idle

My early 850 is fairly new to me (a little over a year now) and it has always had an intermitent fast idle. Most of the time it will idle fine around 1000. Other times (about 70/30 to 80/20) when I come to a light, the idle won't come down below 1800. When this happens, I can hold the brake and ease out the clutch and it will come down and stay down. I also just noticed that if it is idling fast and I move the carb idle screw even slightly, then the idle comes down. When I have discussed this problem, some people are convinced that it is the carb sleeve and others are equally convinced that it is the auto advance. At this point, I would just like to know by symptoms how to determine whether it is the carb or the auto advance.

I doubt if you could tell just by the symptoms.
The fact that fiddling with the idle screw ( it is the idle screw not the pilot mixture screw isn't it?) cures the problem suggests a sticking throttle to me. Maybe there's a burr on the ramp that the idle screw pushes on.
If it's the pilot air screw that cures it, it's probably an air leak through there. They have o rings on them that can wear and allow air past.

Auto advance. Install an electronic ignition and be amazed at the change.

Meanwhile, make sure the slides are synched (both move off the stop at the same time) and adjust the low speed and idle with a vacuum gauge, or, my favorite, a TwinMax.

My '73 850 does much the same thing.

I have a Boyer system, so I don't think it's the ignition advance that's the problem on my bike. I find that blipping the throttle restores the idle to normal 750-1000 range, so I blame it on the carbs sticking a bit. SOmetime it takes a couple of blips. I have new Burlen Amals with chromed slides. I think they should wear in a bit with time. I should probably lube the throttle cables, and/or replace them with nylon lined cables.

The bike was recently serviced and starts and runs fine, except for this one annoyance. I haven't taken the carbs off to lap in the slides because I don't want to mess up an otherwise well running bike.

Sorry, I neglected to say that the carb is a single 34mm Makuni. It is the idle adjusting screw that I am turning. The Norton is an Interstate and the carb linkage takes a pretty sharp bend right out of the carb but blipping the throttle doesn't seem to have any affect on the extra revs.