BBD big bore 2 barrel carb help

The float bowl has to see atmospheric air any time the engine is running .Atmospheric air pressure is what makes the carb work. No bowl pressure means no worky

During the warm up period, and on fast-idle, the low speed circuit is the active circuit.
You can see the mechanical bowl vent activated by the accelerator pump arm. Perhaps when you kick the fast-idle off, the vent slams shut.