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.
