mixes air and
by means of several
. The jets are small holes which allow precisely controlled amounts of fuel or air to flow through them. The amounts are governed by the diameter, length and shape of the jets.
The jets come into action as needed by changes in the
inside the carburettor, caused by different
speeds and throttle openings.
Since the jets are fixed - not adjustable - the mixture is adjusted by directly regulating the fuel or air flow to them.
On carburettors made before about 1974 you can adjust two settings. One is the
of fuel (or on some types, air) going into the
. The other is the engine's idling speed, which is set by adjusting the throttle-stop screw.
The adjustment screws are located differently on different carburettors.
Later carburettors have had to comply with anti-pollution laws concerning exhaust gases.