I think that 2L ford cortina carb has the emulsion tube you are after.
My supercharged A14 has the same "blup" sound when I dump the secondary. (Kind of like a rotary with mechanicalised secondarys) This is because there is no draft operated butterfly above the venturi. Std hitachi carbs have these, as the air speed through the secondary increases, it pulls the butterfly open. This helps maintain manifold vaccuum. As the webers' don't have it, when the big hole is opened, the engine momentariliy loses all manifold vaccuum, and it can't pull any fuel through. Thus the wee stumble as the motor suddenly gets too much air.
Still, it sound quite cool on my engine to have that wee blip, right before you get pushed hard back into the seat.