Description and Explanation of Working.
The illustrations should be referred to for explanation of the following sections. The purpose of the carburetter is to atomise the correct amount of fuel with the air that is induced into the engine, and thus supply a correctly-proportioned mixture at all speeds within the engine's range at all throttle settings.
This is achieved by the selection of the correct size main jet, and main choke bore, in conjunction with the right adjustment or setting of the jet needle and the pilot jet.
The volume of mixture, and therefore the power, is controlled from the handlebar twist grip which causes the throttle valve in the carburetter to be raised or lowered, and the correct setting of the carburetter provides the right mixture at all positions of the throttle valve.
The opening of the throttle brings into action ﬁrst the mixture supply from the pilot system, for idling at slow speed, through the pilot outlet (M). The further progressive opening of the throttle admits air via the main intake and reduces the depression on the pilot outlet (M), but in turn a greater depression is created on the pilot bypass (N) causing the mixture to flow from this opening as well as from the pilot outlet. At about 1/8th of the throttle opening more air is admitted and the mixture is augmented from the main jet (P). The throttle valve cut-away governs the mixture strength from this position of the throttle to about 1/4 open. Proceeding up the throttle range the mixture strength is controlled from about 1/4 to 3/4 open by the position of the needle (C) working within the needle jet (O). The main jet does not spray directly into the mixing chamber, but discharges through the needle jet into the primary air chamber, and the discharge goes from there as a rich fuel-air mixture through the primary air-choke into the main air-choke. This primary air-choke has a compensating action. After about 3/4 throttle opening the main jet is the only regulation.It will be understood from the foregoing that as the main jet only exercises a regulating effect on the mixture strength after the throttle is opened 3/4 of its travel or over, the ﬁtting of a smaller main jet for economy purposes is useless, and can only cause overheating due to excessively lean mixtures at high speeds. As the mixture strength over the greater portion of the throttle range is controlled by the setting of the needle within the needle jet, cases of excessive fuel consumption are usually capable of improvement by adjustment of the needle setting (referred to later) or by replacement of the needle jet and /or needle because of wear on these components.