KEEPING ZENITH
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CARBURETTERS AT
CONCERT PITCH
AS a carburetter is a delicate device it is essential that, before attempting to make adjustments, the operator or driver of a vehicle, should have a thorough
knowledge • of the particular mechaniSm with which he has to cleal and sholdd be nufait, with"the most expeditious Methods of Maintenance. This article, dealing with the standard models of one of the most popular makes, namely, the Zenith, will, therefore, be of more than ordinary interest. The points mentioned are applicable to both vertical and horizontal types.
If general efficiency and freedom from costly stoppages on the road are to be the operator's keynote, then cleanliness in all things connected with the carburetter is
essential. It is advisable to keep the outside of the carburetter free from oil and dust, and at regular intervals thoroughly to clean the interior.
Examine for Signs of Corrosion.
All recent models are equipped with a fine-mesh gauze filter at the base of the float chamber, and it may be readily removed after dis
connecting the petrol pipe. Carefully examine the filter for signs of corrosion, which leads to the formation of small holes, and do not wipe the filter with rag, because this invariably leaves fluff behind it, the best method of cleaning being to swill in petrol,
B33 • If in doubt as to the serviceability of an aged filter it is best always to renew it. The lowet end of the slow-running tube is also fitted with a small gauze cone. Periodically • the "pings under the jets should be 1.-eilloved, preferably with a box spanner, and any sediment cleaned out ; the float chamber should similarly be cleaned after taking out the float.
In cases of a sudden increase in petrol consumption or a report that the engine lacks in power, it is wise to check over other components before examining the carburetter, because it should be understood that the Zenith has no moving parts that can get out of adjustment. In this connection it is worth mentioning that many troubles put down to faulty carburation are actually due to defects rim the ignition system.
Advancing Ignition Timing.
Another point to he observed is always to have the ignition timing set as far advanced as is reasonably possible, whilst frequent attention should be given to the gap at the sparking-plug points, which tends to increase with service. Moreover, attention to the cleanliness and adjustment of the magneto and sparking plugs will prevent such troubles as spitting in the carburetter, overheating, lack of power, etc.
If an air strangler be fitted to the carburetter it is as well to see that it moves freely to the fully open position when the control is released.
After lengthy usage, and particularly when the vehicle is docked for a complete overhaul, it is advisable to recondition the carburetter by returning it to the manufacturer, as the best results cannot be obtained if a certain amount of wear ii\as occurred in the throttle-spindle bearings, needle and seating, etc.
Petrol Leakage.
The correct level or petrol in the float chamber is of importance, as it will affect the running and, moreover, if it be too high, will cause leakage and excessive consumption. When the engine is not running the level should remain 2 mm. below the top of the main jet and main-jet cap. A useful point to remember is that the manufacturer can supply a glass gauge to screw on to the carburetter, enabling the level to be seen at a glance. If too high, the collar , on the needle valve should be unsolder9d and moved a shade farther from the point, and vice versa,
Leakage of petrol from the carburetter is generally due to dirt having settled between the needle valve and its seating, or to these parts being worn. In the former case, wiping the needle and seating with a clean rag and then polishing with metal polish is the most successful cure. Where there are signs of wear, however, the user will be well advised to fit a new needle and seating.
Grinding in the Needle Valve.
It is distinctly unwise to attempt to-grind in a needle valve with emery or similar paste, because such a process will probably ruin the needle and the seating, with the result that it will be extremely difficult to make the carburetter petrol-tight. If grinding be resorted to, nothing coarser than ordinary metal polish should be used.
If the main-jet cover be loose it will cause leakage, so care should be exercised in periodically tightening this part When checking over a carburetter after considerable service, always see that the two small holes in the carburetter body iminediately surrounding the slowrunning tube are perfectly clear, these providing the atmospheric release to the compensating jet. Restriction of these holes would naturally upset the correct functioning of the compensating jet, resulting in heavy petrol consumption and sluggish running.
In the Zenith instrument special provision has always been made to ensure the correct mixture being provided for starting and slow running, and the design of the device used in recent years, virtually a carburetter in miniature, enables the mixture strength to be adjusted while the engine is running and once correctly set it cannot alter. Difficulty in obtaining definite and smooth idling from an engine in good condition can in some cases be traced to the concave cone at the lower end of the top, or adjustable portion, of the slow-running tube having been split by being screwed down unnecessarily hard upon the top of the jet portion.
Experimenting with Jets.
When a new vehicle is handed over by the manufacturer the sizes of the choke tube, main jet and compc13 sating jet fitted in the carburetter are the result of careful:and lengthy tests and rarely require alteration in service.
When the initial newness has disappeared the effect can be tried of using a main jet that is one size smaller than previously used, and if It be found possible to employ this without interfering in any way with the performance of the vehicle, an experiment can be made with a smaller compensating jet. If it be not possible to employ smaller jets, it is evident that the mixture strength is approximately correct.
• It must be remembered that running a vehicle on too weak a mixture invariably results in poor petrolconsumption figures, seeing that the performance is impaired, necessitating more frequent use of the lower gears, whilst burning of the valves and their sea tings is always likely to follow.
Consult the Manufacturer.
In cases where a vehicle is operating in flat country, it may be found that the maximum power of the engine is not called for, and with a scientifically designed carburetter this condition can be taken advantage of and most economical running can be obtained by the use of a slightly smaller choke tube with a corresponding reduction in the sizes of the main and compensating jets employed.
The Zenith company can usually advise if it be considered that an instrument is not adjusted to the best advantage. Heating in some form or other Is essential to efficient carburation and whilst modern induction systems are carefully hot-spotted, or water jackets fitted to lengthy pipes and bends, etc., a number of older vehicles, in which no such provision is made, is still being used. In these cases the heating of the air supply to the carburetter will, undoubtedly, result in more economical operation, provided that it be carried out in a suitable manner.
Carburation and Heating.
The usual practice is to bolt a muff to the exhaust pipe, connecting this to the intake of the carburetter by means of flexible metallic tubing. It is most important that no restriction
be caused at any point to the free flow of air, particularly at the muff itself.
A regulating sleeve is often embodied, enabling the temperature of the air entering the carburetter to be varied in accordance_ with the prevailing weather conditions.
The sizes of Zenith jets, both main and compensating, vary in fives, whilst the number stamped on the choke tube indicates its internal diameter in millimeters.
Zenith Service.
The manufacturer is always willing to loan optional jets for testing purposes, but as all jets are carefully calibrated on tiowmeters to extremely fine limits, it is essential that a jet should never be reamered out or hammered up. By doing this one loses all bases to work upon and consequently it is always advisable to obtain on loan some correctly calibrated jets from the Zenith CarbureVer CO., Ltd., 40, Newman Street, London, W.I.