zedzil wrote:
Disconnected the throttle linkage from the carburetor and ran engine while operating the throttle arm on the carburetor manually and the problem persists.
Sounds like you have the source then. Maybe the accelerator pump piston (seal) is getting hung up in the accelerator pump cylinder (assuming it isn't a diaphragm type pump)?
At idle there is a lot of suction force on the butterfly so there might be some problem there that you can't feel statically. If the overhauler removed the butterfly from the shaft it may have been replaced in a slightly differently location than it was before (tolerance between the screws and the butterfly) and could be dragging on the housing, but only noticeable under vacuum due to wear between the shaft and the bushings.