Carburettors are classified into two types: fixed jets and variable jet instruments, also known as fixed and variable choke. When the engine is turned off, a simple fixed jet carburettor displays the fuel level. When the engine is turned on, fuel flows from the float chamber to the emulsion tube, where it is mixed with a small amount of air to increase engine speed before passing through the nozzle and into the venturi.

With the throttle butterfly closed, the engine continues to run at tick-over speed through the idling jet and the progression tube. When the throttle is quickly opened, the accelerator pump provides a brief enrichment of the mixture (via the top nozzle in the venturi).

The carburettor with variable jets SU The amount of fuel that passes through the main (and only) jet into the airstream is controlled by a tapered needle attached to the vacuum-operated piston. This, in turn, varies with engine speed and throttle opening.

A British Leyland O-series engine with an SU carburettor. The dashpot can be seen right next to the air-filter box. Check the oil level in the dashpot with the variable jet type of carburettor by removing the damper; if necessary, top-up with fresh engine oil. If the carburettor has a piston lifting pin, it can be used to check the fuel/air mixture; if it is correct, the engine should accelerate briefly when the pin is lifted. Under the unit of some SU carburetors is a mixture adjusting nut. To change the setting, turn this a little to the left or right. Release the clamp that connects the throttles of the twin carburetors before synchronizing them.

To facilitate mixture adjustment, a quartz-bodied Colortune spark plug is used. The color of the combustion flame can be checked and the mixture adjusted until it is satisfactory. A filter within a glass bowl is sometimes used in mechanical fuel pumps. By loosening the retaining screw and releasing the wire clip, this can be removed. Using a paint brush and gasoline, clean the filter.