The needle is a metering device in the main circuit and can only work when the main circuit is working.
The idle circuit flows fuel because of the pressure drop ("vacuum") on the back side of the butterfly/slide. Once that slide starts to open, the signal on the idle circuit drops. But, there is not enough air flow speed yet, to start the main circuit, so fuel has to come from someplace else. The idle circuit is still supplying fuel, but it is not enough for the increased air amount, so the transistion fuel is added to it.
The transition circuit is usually a few holes or a slot, right behind the butterfly/slide. In fact, some carbs use the butterfly to partially meter the amount of transition fuel (Holley, among others). With your air cleaner off, open the carb and look for some holes right behind where the butterfly/slide seats when closed. That is, the holes will be closer in toward the cyl. head and will be on the floor of the carb throat.
The jet that controls the fuel to this circuit might be buried in the carb somewhere, behind a pressed-in plug, much as the idle mixture screw is covered up. It's possible that the passageway can only be seen with the float chamber removed. It's also possible that the holes themselves do the metering, but most likely there is a brass insert jet. Never the less, the transition circuit is in there somewhere.
