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Improving Carburetter Efficiency
Page 103
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A New Type of Zenith Carburetter Designed to Give Economy Coupled with Maximum Power ONE of the great difficulties in designing and tuning a carburetter is usually to arrange that the last quarter of the throttle opening, gives an increase of power and speed. Generally, one finds that the engine, Epeed rises as the thottle is moved from the idling position until the butterfly valve is three-quarters open; beyond this point there is little or no response. The process of tuning can overcome this difficulty at the expense of economy, however. Obviously for certain types of commercial service, where full throttle is needed for prolonged periods and economy is vital, this design should prove of value.
In the ordinary course of events very few drivers require maximum power more than two or three times a day; economy up to three-quarters throttle is_ more valuable. Bearing this in mind, the Zenith Carburetter Co., Ltd., 40-42, Newman Street, London, W.1, has adopted the design to which we make reference here—the 116-type instrument. It is a development of the pumptype carburetter which we described in our issue gated May 7th. The illustrations on this page show the principal point of difference—the economy needle. The main jet is in its ordinary position and is calibrated to pass the maximum amount of fuel required at full throttle opening. Petrol flows to it through a regulator mounted in the base of the float chamber. This regulator, or economy jet, passes slightly less fuel than the main jet will take. This suffices for work up to three-quarters throttle.
When maximum power is desired the accelerator pedal is fully depressed and a cam on the throttle spindle comes into contact with a lever, which, in turn, raises the needle, opening a special passage to the main jet. The passage in question allows the full flow of petrolto the jet without any impediment. Therefore, when the needle rises full power is available and the economy jet is short-circuited. This action is indicated in the accompanying diagram.
The 42 mm. size costs £9, whilst the 48 mm. carburetter is priced at £10.