In this three to five jets are provided in the carburettor. A cap E serves to open different jet passages in turn. First when the throttle opening is small, a smaller amount of suction is applied and only nozzle I is then open to engine suction. The fuel is now provided by the accelerator pedal is pressed, the outlet valve is opened and the fuel is forced out of the, acceleration jet. When the pedal is released, however, the piston moves up thereby sucking the fuel in from the float chamber. The pump is thus ready for next discharge.
Influence of Weather
As already explained simple carburettor can be set to deliver correct air fuel mixture only at a particular speed. Further if the carburettor is set for a particular weather, for instance summer, then in winter months it will give weak mixture because in winter the air density increases to a large extent as compared to the increase in the fuel density. As a result the mixture becomes
weak. Similarly, a carburettor set for winter months will give too rich a mixture in the summer.
In the modern carburettors, climatic control devices are provided. These control the mixture strength by varying either fuel jet area or the air intake.
Influence of Altitude
At higher altitudes, the air density decreases due to fall in atmospheric Pressure. There is another factor contributing towards increase of air density at high altitudes and that is the decreased temperature. But decrease in density due to fall in atmospheric pressure outweighs this factor and net result is an appreciable decrease in air density at higher altitudes. Due to this a simple carburettor which is set to give correct mixture at lower altitudes will give a comparatively richer mixture at higher altitudes.
A method which is used to rectify this defect in some Zenith carburettor. Two pipes AB and CD connect the top of the float chamber to the air horn. S is a valve in the pipe CD. For ordinary low altitudes, valve S is closed and thus the fuel in the float chamber is under atmospheric pressure because of pipe AB. As the altitude increases valve S is opened gradually and by so doing engine suction is applied to the float chamber, thus decreasing the pressure here which results in decreased fuel supply and hence a weaker mixture at higher altitudes, which is desired.
Icing Trouble
As the fuel is atomised and evaporated in the carburettor, it causes cooling of the surrounding areas. In localities which are cold, there is thus always the danger of ice formation and choking of the venturi tube. Some of the methods used to prevent ice formation are:
(i) By heating the carburettor idle ports and throttle valve area by means of the engine exhaust gases.
(ii) By providing water passes in the carburettor body. The engine cooling system provides the hot water for heating the carburettor, thus avoiding formation of ice.
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