If the gizmo you are describing is near the top of the carb on the outside with an approx. 2\" long x 1/2\" stamped aluminum plate covering it it's the thermal compensator. It was a great idea when your bus was new, raised the idle speed and enriched the mixture when it was cold, the solenoid thingy in there is a heater and even if it still works the seals on the bimetal valve are always done for.
Take it off, box and all (two or three screws holding it on), and seal up all the holes with silicone (including the screw holes), for the odd day when it might have helped you can live with warming your bus up 10 more minutes and forget about the hassle it causes with erratic idle mixtures. Don't forget to tape up the wire that went to it.
