Auxiliary Air Valve
The auxiliary air valve is mounted above the secondary valve and is used to control the timing of when the air flows into the secondary.
The auxiliary air valve should not be modified as many have been. They are application specific so moving an AFB from a Buick to a Pontiac may give you different results. A bog when opening the secondary can be caused by having the wrong air valve (or carburetor), or perhaps a modified air valve. Short fuel supply to the secondary could have the same effect.
Auxiliary air valves are not available new and we have no way of knowing which one goes in which carburetor.
Linkage position when the secondary is closed (primary is also closed).
Linkage position when secondary is starting to open.