How it works & part names

The carburetter proportions and atomises the right amount of petrol with the air that is drawn in by the engine because of the correct proportions of jet sizes and the main choke bore. The float chamber maintains a constant level of fuel at the jets and cuts off supply when the engine stops.
The throttle control from the handlebar controls the volume of mixture and therefore the power, and at all positions of the throttle the mixture is automatically correct. The opening of the throttle brings first into action the mixture supply from the pilot jet system for idling, then as it progressively opens, via the pilot by-pass, the mixture is augmented from the main jet, the earlier stages of which action is controlled by the needle in the needle jet. The main jet does not spray directly into the mixing chamber, but discharges through the needle jet into the primary air chamber, and goes from there as as a rich petrol-air mixture through the primary air choke into the main air choke. This primary air choke has a compensating action in conjunction with bleed holes in the needle jet, which serves the double purpose of air compensating the mixture from the needle jet and allowing the fuel to provide a well outside and around the needle jet, which is available for snap acceleration.

The carburetters usually have a seperately-operated mixture control called an air valve, for use when starting from cold, and until the engine is warm; this control partially blocks the passage of air through the main choke.

Diagrammatic section of carburetter showing only the lower half of the throttle chamber with the throttle a little open and the internal primary air passages to the main jet and pilot system.

Fig 3.