in a clockwise direction, which richens the mixture, after which various positions must be tried until the engine runs slowly without any hesitation and will start readily. After having discovered the correct position by trial and error, the adjuster is firmly locked by a nut below the knob. There is no standard adjustment for the knob regulator, the position of which will vary with individual engines. As a rule, however, it is somewhere between the fully closed position and two complete turns unscrewed. When the slow-running jet is correctly set, the well in which it is placed will remain about half-full of petrol, thus providing a reserve for acceleration. When the throttle is fully opened, this fuel rushes through the passage (7), to balance the air passing around the compound jet. The Zenith compound jet, it will be seen, consists of two distinct jets, the central one, called the main jet, being of the usual type and fed direct from the float
to the air through the hole (3). I?roin the compensator jet and its well the petrol passes up the side of the main jet, but as the supply from the compensator cannot increase with the engine speed, while the air passing through the carburettor does, the mixture given by this jet becomes gradually weaker. This also provides the necessary correction. Moreover, the reserve of petrol remaining in the well when the engine is running slowly greatly assists acceleration, as it passes very quickly through the main jet cover and balances the sudden rush of air passing through the carburettor directly the throttle is opened.
Tuning for ” Revs. ” and Power. Though the Zenith carburettor is so simple in its action, considerable care is necessary in the processes of fitting and tuning. As already stated, the carburettor is adjusted by determining the correct sizes for the
chamber. Owing to the fact that, under suction, the flow of petrol from this jet increases more rapidly than the flow of air, this jet gives a mixture which gradually becomes richer as the engine speed increases. The outer jet, which is really a continuation of the compensating jet (6), has an opposite effect in that it supplies a gradually weakening mixture as the speed increases. This is due to the fact that it only delivers the amount supplied by the compensator, which, as explained, is unaffected by suction. In other words, when the main jet is passing too much petrol, the compensator is giving too little, and vice versa. Thus the two jets, working in combination, maintain an ideal balance and the result is a constant mixture at all engine speeds. The full action of the compound jet can be more easily grasped by considering that of the compensator jet, which is supplied from the float chamber but feeds petrol into the well above it, and is quite unaffected by the action of the carburettor itself, as both the jet and the well are open choke tube, the main jet and the compensator. The purpose of the choke tube is to determine the correct velocity of air passing the jets in order to obtain the best mixture at all speeds. The main jet has the greatest influence at high engine speeds, and the compensator, which corrects the irregularities of the main jet, exerts the greatest influence at low pulling speeds that is, when the vehicle is pulling hard or climbing hills. After having regulated the slow-running adjustment in the manner already described, the engine should be allowed to run until it has become thoroughly warmed, which condition is very desirable before any attempts are made to select the sizes of the choke and the two jets. When the choke tube is too large, it will be impossible to get the engine to accelerate rapidly, irrespective of the size of compensator jet. Expert tuners can pick out the correct size of choke tube for practically any kind of engine without taking a road test, but this only comes by long exerience a,nd constant practice. Ac celeration tests can be made by the aid of a Wimperis accelerometer, or by means of an ordinary stop-watch used with the speedometer. If, when the accelerator is fully depressed, the engine slows down or stops completely, a larger compensator jet may be tried ; but, should no further improvement be noted, it will be necessary to remove the choke tube and replace it with
• one of smaller internal diameter. As a rule, the reduction of choke tube diameter tends to a heavier petrol consumption, so that this should be taken into account in order that a very rapid” pick-up” is not secured at the sacrifice of economy. When the choke tube is too small, the acceleration will probably be excellent, but the speed of the vehicle on the level will be inadequate, while a tendency for the engine to choke itself—coupled with a petrol-laden exhaust—will serve as clear indications that a larger size of choke tube is necessary. Any changes in the size of the choke must be accompanied by corresponding changes in the jet sizes.
Selection of Main Jet. The selection of the correct size of main jet must be carried out by tests similar to those mentioned above. A jet which is too large will cause the engine to choke much in the same way as when a very small choke is used, and when any demand for power is made the engine will run irregularly or perform a trick commonly
known as” hunting.” The best results will be obtained from a size of main jet which gives the best maximum speed on the level, and, should two jets give the same performance in this respect, the smaller should be chosen on the score of economy. The main jet is too small when the acceleration is feeble and when popping occurs in the carburettor as the throttle is opened. This popping occurs at irregular intervals, and the engine will, in consequence, fail to develop its maximum power. The size of main jet should be increased until the popping disappears.
Selection of Compensator Jet.
As already explained, the action of the compensator exerts the greatest influence at low-engine speeds, or when the engine is pulling its full load on an incline. Therefore, the selection should be made when the vehicle is pulling up a hill with the engine speed as low as can be maintained. If the compensator is too large the engine will run irregularly, and the ” hunting ” which takes place at high speed when the main jet is too large will be found at low speeds if the same fault exists in the case of the compensator. The size of the compensator, therefore, should be reduced until the engine runs smoothly and the exhaust is regular. The symptoms developed when a compensator is too small are misfiring and stopping of the engine.