You have two idle speed screws, a fast idle that should be in play while the choke plates are closed and or during some time while they are opening. The other is the Curb idle speed screw, both control the throttle plate position.
If your fast idle speed screw is set too shallow to reach the fast idle cam or so deep that it cannot 'climb' onto it, either will prevent the function that was intended, faster idle during the warmup.
Likewise the Curb Idle speed screw set too deep could allow the same issue... choke plate in play= slow idle, choke plate open=faster idle speed, if the fast idle screw is out of position.
The cure is to set the screws.
Which "Weber" have you mounted?