Fast idle was common issue with circa '72 Chevy 2 bbl and 6 cyl 1 bbl’s. Issue was 2 fold: loose throttle body to bowl gaskets and worn throttle bodies which only stuck under vacuum. Fix was a new throttle body and a carb o’haul (while not absolutely needed, turning stuff upside down, etc, made an overhaul logical for reasons of dirt and come backs).
Now our SU’s:
Chasing a fast idle on our new carbs causing also a Moss shift issue (hanging fast idle speeding up the input shaft too much). Never stuck cold with engine off. All the stock springs under the dash or before the carbs themselves just shut the linkage to the free play point. The 3 springs attached to the “Y” brackets take the throttle spindles to the stops the rest of the way. This is a small distance to bring the little levers to the fast idle choke stops. When under vacuum ours wouldn’t seat - probably due to the listers who talked about neoprene, cork, bent shafts, etc. Well, you can also ‘clock’ the spring attachment levers. Since there is an ideal geometry and limited options you need to have the springs clear the lever nuts. Before you replace seals which may bind I’d suggest running these levers to put the max tension on these springs. They are so weak that it won’t increase your effort by much, but will put a bit more tension on those little levers.
It’s incredible how little the throttles need to be open to give a 1 600, rpm idle.
I had the same problem , fitting HS8’s to my MK2 , the springs just would not shut off the butterfly’s, as a temporary fix I added another Y bracket I had laying about , works so well I have left it on ! 
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Seems a common problem. With all of those moving parts and expecting nothing the get gummed up or worn is a bit wishful… I had it on my 3.8, now on my 4.2. I’m not digging into it until after the rebuild though.