I presume those carbs are 28mm.
That cover is redundant on those and is used to house a diaphragm on the 26mm version .
Google vm26mm pumper carbs or look on WG carbs web site for full details..
That's the vacuum sensor. the vac sensor diaphragm has a brass plunger that pushes on a steel ball. I think it allows fuel to flow to the AP when the engine is running, but stops the flow when the engine stops. it's kind of a rube goldberg epa type of thing
On some models the device was disabled at the factory, as on the 81 650. mine didn't have the cover & the orifices on the flange aren't factory drilled or anything. kinda puzzling at first, especially since I didn't know anything about carburetors at the time. the rebuild kit had this triangular thing that wouldn't fit. doh.
