Weber Synchronus with stock Kickdown

Looking at your return spring, is it pulling directly through the throttle shaft near idle? The axis of the spring always has to be off to one side of the throttle shaft to have any leverage to close it.

Check in the throttle body and see if the throttle plate is closing against the side of the throat; it should always have a small air gap edge of the plate and the throat. If not and it is closing fully against the side of the throat, the idle will tend to be erratic. You have to make other adjustments to get the idle lower.