[Hanuman]

Wouldn't this setup use an Idle stabilisation valve?

If so...Probably shaped like a cylinder a few inches long by an inch or so in diameter, roughly.
They can get a bit gummed up over time, requiring a light clean, or more seriously a fault internally, maybe requiring a light replacement.
There have been a couple of threads on a similar vein recently.

How long has it been like this?

At idle, the throttle should have no load on the cable at all, and shouldn't be pulling on the throttle plate.