Sounds like the throttle stop screw is not touching the throttle shaft allowing the plate to jam in the bore under vacuum.

The throttle stop screw looks like it should adjust idle speed but its purpose is to keep the throttle plate off the walls of the bore. The screw should be backed off til it doesn't touch the throttle arm then be adjusted up to the arm till it just moves it.

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Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician