You will see a slot opening just above the throttle on the throat of the carburetor. This is the idle port which opened into the idle fuel passage.
When the throttle valve starts to open, this port is uncovered and it will begin to supply fuel mixture to the engine. This is in addition to what is already supplied through the idle screw port.
This additional mixture will furnish enough extra fuel to allow the engine speed to increase to the point where the main discharge nozzle will function. Instead of the slot-shaped opening, some carburetors use a series of small round holes. As the throttle opening grows wider, it keeps uncovering extra holes.
