Main System

With the throttle plate fully open, the carburetor main fuel system takes effect. The small amount of air is supplied through the air jet position on the outer face of the carburetor. This air travels through the horizontal passage that turns downwards through the inclined passage to the emulsion tube chamber where it mixes with the fuel drawn through the main fuel jet. The mixture of fuel and air enters the carburetor Venturi through the top of the emulsion tube and here it mixes with the additional, main quantity of incoming air. Because of the tapering constriction in the middle of the Venturi, air flow increased velocity causes low pressure that draws the fuel through the main jet and emulsion tube upwards.
For a small engine, the main system air fuel mixture is still rich.

See movie below: