Fuel is fed to the carburetor through the main metering jet C.
As the throttle valve is opened (higher speed), the velocity of air going through the bore creates a suction at the main discharge. This causes fuel to enter through the main jet C and mixes with air through jet K. The fuel air mixture travels through nozzle L and into the carburetor bore.
Adjusting M clockwise will leans the mixture. Counter clockwise will richen the mixture.
About 3 turns of the metering screw should get optimal speed.