That carb does not use a fast idle cam like some others. There a very fine-toothed ratchet assembly in the choke housing that advances the throttle linkage to increase the idle speed when the choke is applied. Inspect the teeth for wear or looseness- - - -it's a flimsy-looking exception to the usual excellent Carter carb design.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!