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32

TUNING CARBURETTORS FOR SPEED AT COMPETITION EVENTS. Further articles in this series will deal with the tuning of individual instruments, and will he based on the author’s practical experiences.

T is often said that where sheer speed is alone con cerned nothing more is needed in the way of a car

burettor than an arrangement for admitting as much fuel to the engine as it can possibly swallow at its maximum numbr of revolutions per minute, but this does not apply in the case of the sporting motorist, whose demands more nearly approach the normal where the correct atomisation of the fuel is concerned.

From a theoretical point of view the subject of carburation is one of intense interest, embracing as it does not only the formation of a correct mixture for all engine speeds and conditions, but also the proper distribution of the fuel to the cylinders. Thus the shape and character of the induction pipes, the number of carburettors employed per engine, and such matters, come up for consideration ; but in dealing with the subject from the reader’s standpoint, we have to approach it from a rather different angle.

For example, the average sports car owner is not generally disposed to go to the trouble and expense of duplicating carburettors on his engine, or in having special types of induction pipes made, as this in many cases would involve extensive alterations to the engine and the arrangement of the accessories, but it is quite likely that he may be anxious to become acquainted with the best methods of securing the utmost efficiency from the particular form of instrument fitted to his car whilst if he goes a little farther there is the possibility of improving car performance by exchanging the existing type for one of more recent design or improved pattern.

Though similar in principle, many of the carburettors of to-day possess certain little peculiarities in the direction of tuning, the details of which will be of interest to those accustomed to perform the operation of preparing cars for trials in their home garages, whilst a thorough knowledge of the instrument and its method of operation is very valuable in getting the best results from a car. The bete noir of every driver, known as the “flat spot,” can almost invariably be overcome by a careful gradation of jets and chokes, and though it is practically impossible to ensure that a single setting will give the maximum power and acceleration combined with • the greatest economy of fuel, by a close knowledge of individual carburettors it is a simple matter to arrange for two or more settings to serve for different classes of driving. Thus on long reliability trials, where one does

not want to pour petrol into the engine with a carburettor adjusted for a full power setting, required for hill climbs near the end of the route, the run can be done economically on one setting, and a quick change of jets and choke tubes will enable the driver to put up a good show when the rough country is approached.

It has been our experience that the number of drivers who take the trouble to really understand their carburettors is comparatively small, and whilst many speak glibly of diffusers and so forth, their actual operation has not been fully investigated.

Carburettors of to-day are vastly improved since the time when a single jet device was considered adequate to supply the demands of the engine, and the introduction of the principle of compensation has rendered the work of tuning for maximum efficiency very interesting, if not actually complicated.

Comparing Carburettor Results.

When it comes to making actual comparisons between the results obtained from different forms of carburettor, one encounters various difficulties, and the best that can be done is an approximation of results in determining which particular form of carburettor is best suited for the needs of an engine. A very slight difference in atmospheric conditions will altogether upset comparisons. The condition of the roads, to say nothing of possible variations in the performance of the engine, all tend to produce results that may not be exactly fair to an individual instrument.

At the same time, one can form a very fair idea as to the suitability of a carburettor to a given engine, if certain definite precautions are taken. The best time to try a set of carburettor experiments is when the engine is new, but has been running for a sufficiently long period to work off its original stiffness in the bearings and for the working parts to have settled down to a normal state.

Instrument Board Equipment.

Even though from a purely theoretical point of view one cannot hope to get the closest results by comparisons, the interest in carburettor testing will be greatly enhanced if the car used for the purpose is provided with a fully equipped instrument board. For the purpose of collecting the data to be published in a short series of articles on Tuning Carburettors, to which the present notes form an introduction, the four-seater Frazer Nash car shown in the accompanying photograph was equipped with a fairly complete set of instruments, which, as shown herewith, is fairly well selected for the job. The range includes a clock, a radiator thermometer, used in conjunction with a Boyce radiator shutter, a

Tapley gradient meter, a Metreta petrol gauge, the usual switchboard, an oil thermometer, oil pressure gauge, a revolution counter and a speedometer. A further instrument not fitted on account of lack of space was an aneroid barometer, but as this instrument is not provided with the necessary mechanism for changing a moist, foggy atmosphere into a dry, clear one, its utility from a carburettor testing point of view was considered to be somewhat limited, and its services were threfore dispensed with.

For the benefit of those of our readers who regard the employment of many instruments as a chronic form of ” gadgetitis,” we may give a few reasons why each one was selected, and indicate the point of using them in connection with carburettor experiments.

Controlling the Engine Temperature.

It is an accepted fact that no attempt should be made to make any adjustments until an engine has warmed up sufficiently to ensure a correct vapourisation of the fuel, quite apart from the numerous other advantages of maintaining the correct water-jacket temperature at all times. The common practice of fitting a thermometer to the radiator cap is all very well in its way, and does provide a certain safeguard for the ordinary driver, but its use is extremely limited unless it is used in conjunction with a radiator shutter.

Even when a shutter is used, it is far more convenient to have the thermometer directly under the eye of the driver, hence the advantage of having this instrument mounted on the dashboard, where with proper illumination it can be seen by night as well as by day..

Having tried several forms of shutter in daily use, we find the best results are obtained from the type manufactured by The Benjamin Electric Company, which, in addition to being extremely efficient, is certainly the least expensive on the market.

Additional Dashboard Fittings.

A dashboard petrol gauge of the Metreta type is also indespensable in testing fuel consumption, as the needle

working over a large dial registers the amount of petrol used with great accuracy.

The new model of Tapley Gradient meter is an instrument which certainly ought to be included in the equipment of any car that is used for something more than a mere means of transport from place to place, and by studying the possibilities this device provides, a very large amount of practical information can be obtained. Not 6nly will it register the up and down gradients, but with a little practice it is possible to get a very good reading concerning the acceleration capabilities of the car, and the actual de-celeration when the brakes are applied. Thus the instrument gives s better idea of the performance and mechanical condition of the car than can be obtained in any other way.

An oil temperature gauge is a fitting seldom found as a car accessory, but is nevertheless of great utility, as it indicates the condition of the oil as well as serving as a warning should any of the bearings develop undue overheating.

A revolution counter and speedometer are of course indispensable and with a full equipment as described above, one is in a position of being able to make very fair comparisons in carburettor performance. The various well-known carburettors and methods of tuning will be dealt with in subsequent issues of this journal, and it is hoped that the information will be found valuable to our readers.

H. S. Perrey to Ride for Ariels.

H. S. Perrey, the well-known competition rider, who has performed so successfully on B.S.A. machines, will ride Ariel machines during the coming season. It will be remembered that he rode a 500 c.c. o.h.v. Ariel combination in the recent Six Days International Trial, while a more recent achievement of peculiar merit is his record drive of a similar outfit up the Snowdon Mountain Railway. On this occasion Perrey broke the solo record for this climb.