I encountered a throttle sticking problem recently, and although I doubt this is your problem as you say that your carbs were recently rebuilt, you might check it out.

On the rear carb, the screws locating the butterfly to the throttle shaft loosened enough that the butterfly could shift laterally enough to bind in the throat and not close entirely, causing a higher idle. (Blipping the throttle would sometimes, but not always jostle it back into temporary alignment and the correct, lower idle.) It required removing the carb in order to accurately locate and tighten the two screws securing the butterfly to the shaft. Problem solved.