Over-revving idling

The advance retard is worn out/stuck.
If there is no choke and it obviously starts then it is running too rich as well.
Colour tune-tells you what you already know.

Hello Richard - Did it ever idle properly ?   You don't mention jet sizes, needle position etc so I presume it was running OK previously.  Is the float needle sticking perhaps or could you have a damaged float ?  Is it a coil ignition or magneto model ?  Cheers, Howard

A good test for air leaks into the inlet is to have the bike idling (as best you can) and spray WD40 or something at the joints and studs.

The engine note will change if the spray gets into a gap.  Usually slows down.

I certainly wouldn't rule out a dodgy auto advance though, if it has one.

Do you actually hear the slide hitting the bottom when you shut the throttle, with the throttle stop screwed out?

... I have a similar problem with my Electra, which will only (approximately) idle with the air screw fully in. I've checked for air leaks but am wondering if an internal fuel passage is blocked. Monobloc carb (original).

I had what seems to be the same problem with my Navigator, slide closed right down, unresponsive to air-screw adjustment and no air leaks.
Turned out the needle had been messed about with, tried another needle, the same.  Then another and all is well.  Seems I had two dodgy needles.
Out of curiosity, I changed the needle position, the high tickover returned and the carb, again, became unresponsive to adjustment.
My thought is that the needle is in the wrong position based on the above?

The two likely culprits are as already noted. Ignition advanced at idle (for whatever reason ) and carb fault. I recently fitted a recon carb to my 99  .  The carb is too big and  is set up for another make and capacity . it still manages to start perfectly and idle as slowly as anyone could  possibly  want.  (Amal and Norton both  warn you that a  very slow idle is not good for pickup or oil supply).  Even with a good carb its necessary to get the engine  to run on the pilot circuit and not be fooled if its getting into the bypass and needle jet territory, where no fiddling with mixture will work.  We are told that the carbs were designed and compensated to work with a sloppy slide from new. Probably because they could not be relied on to stay round for long. My carb has about half the slack of a new carb and its a pleasure to work with.  Mr Bratby did a good job .I just hope it will stay that way after countless heat cycles and if someone (!) slightly overtightens the mounts.

Too advanced makes it idle fast.

Possible that the old slip ring still in use and has moved?. I am going to make the same mod on my PR 650 . Converting a manual mag to auto AR .  To improve the spark consistency.