Annular boosters on a Tunnel Ram

I am quite happy to have a civilised discussion with you about emulsion.
I am not sure what you mean by function of emulsion above the fuel level?

The entry to the booster is 1/4 -1/2" above the fuel level in the fuel bowl, for all venturi style carbs & this is the theoretical level in the main well that feeds the booster. This is called the spill height. In a running engine at idle, the fuel level in the main well is actually slightly lower than this because nearly all American carbs take idle fuel from the main well. The only American carb I am aware of that does NOT supply idle fuel from the main well is the Comp Series TQ. It, like some Weber & Dellorto performance models, take idle fuel direct from the float bowl. I assume this is for more accurate metering of the main system.

To get fuel to the booster entry, the fuel has to be lifted 1/2". What does does that? Atmospheric pressure acting on the fuel in the bowl pushes out the fuel in conjunction with suction/depression at the booster nozzle. It should be obvious that any bleeding off [ MAB ] will delay the start up of the system [ the straw example ].
Air is drawn through the MAB which aerates/emulsifies the fuel in the main well. This makes it easier to lift the lighter emulsion as compared with solid fuel.
A smaller MAB starts the flow earlier because less signal is lost [ or bled off ] & a bigger MAB starts the system later because greater vacuum [ more air speed, higher engine rpm ] is reqd to overcome the signal lost to the bigger MAB.