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Depress the gas pedal, this input allows the choke butterfly to set, the fast idle cam to rotate into position and causes the accelerator pump to discharge fuel into the venturi.

As the engine spins, fuel pressure is provided and vacuum is created. The fuel enters the float bowl through an inline filter. The filtered fuel passes through the float needle jet. As the fuel level rises in the float bowl, it fills the accelerator pump. A float connected to the float needle rises and shuts the fuel flow into the carburetor off. (When cold) This vacuum pulls air from the heat stove on the exhaust manifold into the air cleaner snorkel. (When hot) A vacuum operated damper door (on the snorkel) closes and bypasses the heat-stove air, pulling air directly into the snorkel. Air enters the air cleaner plenum. The air is then filtered. The choke, depending on the temperature (if cold) will block air from entering the carburetor. The fast idle cam holds the throttle open to increase engine RPM. A bimetallic coil on the intake manifold connected to the choke with a rod expands and contracts. This opens and closes the choke butterfly valve and rotates the fast idle cam. As the air passes into the top of the carburetor's venturi, Bernoulli's principal is invoked. The airspeed is increased and pressure decreased.
Inside the venturi are ports where fuel is introduced. These ports are jetted. Fuel is supplied to these jets from the fuel bowl. Mixture screws are used to meter a specific amount of fuel through these jets. The air is pulled and passes the throttle butterfly valve. In this deceased area of pressure, fuel is sucked out of the ports. The fuel-air mixture now enters the intake manifold, where it's channeled into the cylinder heads and awaits intake valve opening. Upon opening, the mixture is pulled in the combustion chamber. As the engine starts, vacuum increase.
This increase in vacuum operates a choke pull off. This opens the choke butterfly enough to allow more air to pass until the engine temperature operates the bimetallic coil. As the bimetallic coil heats up and the choke opens, the fast idle cam is released as the gas pedal is depressed. The accelerator pump adds a burst of fuel for acceleration purposes.