ATH.L;N in the course of some race or competition, mechanical trouble forces one to retire, the more trifling or obscure the cause the greater one’s disappointment and chagrin.

A major fault in one’s car, such as a broken connecting rod, even though it wipes success from a driver’s grasp can be accepted with resignation ; not so, if the reason for withdrawal is ultimately traced to dirt in the fuel supply or a broken feed pipe, for it is then that one realises that a little more care, a little more forethought would have obviated the trouble.

Nearly every car-owner has experienced, at some time, the usual choked jet, and it must often appear that this occurs at the most inconvenient moment—generally when one is in a hurry—and if this is the case, it is obvious that the reason is that, in being driven hard, the engine requires the fuel at a greater rate, and thus any accumulated dirt in the tank or pipes is suddenly drawn up by the increased suction and causes a stoppage.

Fuel starvation is, perhaps, more common in sports machines than in other classes of cars, and when the tank is fitted at the rear, the obstruction, air-lock or whatever the cause may be, is sometimes difficult to trace. Rear tanks are used for two main reasons—to obtain a lower centre of gravity and to give a greater supply of fuel. Obviously with the rear tank some means of feeding the petrol to the carburettor is called for, and it is in these methods that the last season or so has seen a great deal of change.

On normal cars the vacuum tank held sway for many years, but it has marked disadvantages for sports car use or for any car which is continuously driven hard. The principle of the vacuum tank, as will be known to all motorists, is that the suction on the induction pipe is utilised to raise petrol from the rear tank to the vacuum tank on the dash, whence it is fed by gravity to the engine.

The maximum suction occurs when the throttle is nearly closed, while when driving all out and requiring the maximum supply of fuel, the suction is negligible.

This results in fuel starvation unless the engine is frequently throttled down to get some fuel in the vacuum tank.

The majority of racing cars of a few years ago ensured their fuel supply at all times by means of pressure feed. In this method, of course, an airtight tank and filler cap are necessary, while the air pressure is usually supplied by a hand operated pump mounted on the dashboard and operated by the mechanic. Such a system, while excellent for racing purposes, due to its reliability, is too crude

for standard use on sports cars in the hands of the normal motorist, who has no wish to start manipulating pumps every time he starts off in his car. This had led to the development of the mechanical or electrical fuel pump as a positive means of supplying the carburettor, and it has proved itself to be

reliable and compact. What is more, it has shown itself equal to the strain of racing, and is now used on the majority of racing cars, either alone or in conjunction with a pressure system as a reserve.

If the two systems are used the airtight filler cap must be supplemented by a tap on the pressure line, which can be opened to the au, and so avoid the creation of a vacuum when running on the electric or mechanical feed.

The electrical system seems to enjoy the greatest popularity at the present time, but there is a sign that other systems will also become common, the hydraulic pump operated by the lubrication of the engine being an example. This type is being developed by the S.U. company, whose carburettors are used on many racing and sports cars.

This fuel device consists of a small reciprocating plunger pump driven by a hydraulic reciprocating motor worked by the engine’s oil supply. This, therefore, provides a positive safeguard against damage due to lubrication failure.

The pump is inserted in the system in such a way that the whole supply passes through on its way to the bearings. The motor piston consists of a sleeve with a large aperture in the middle. This is closed by a valve which is connected to the petrol piston by a piston rod passing through a guide which separates the two cylinders. Both the pistons are supported by coil springs, that under the petrol piston being the stronger. Oil enters above the oil piston and forces it down, carrying with it the valve and petrol piston.

During this stroke, petrol is transferred from below to above the petrol piston by way of a ball valve located at the side of the petrol cylinder. Near the bottom of the stroke the sleeve is arrested by a washer located near the bottom of the oil cylinder, and the valve is forced by the oil away from its seating in the sleeve. The pressure on each side of the sleeve is now equalised, and its spring throws it

upwards and the valve follows it together with the petrol piston, which now expels petrol from the top union and sucks a further supply through the side union, via the petrol filter and through the ball valve located below the petrol piston.

The upward movement of the petrol piston and oil valve is controlled by the demand for petrol, and when this reaches the top the valve closes and the cycle repeats itself.

Although not in such general use as the electric pump, it has been thoroughly tried out, and was used on the S.V. Morris Minor which did an official speed of 101.96 m.p.h. at Brooklands last year.

The better known pump made by this company is the S.U. Petrolift, which is really a combined pump and gravity tank. The upper part of the instrument is a small tank, and the falling of the level in this automatically starts the pump and keeps it going till the level is restored. This ensures a constant gravity supply, and in a case where a dash tank is being removed and a rear tank fitted, no alteration will be required to the carburettor needle mechanism.

This instrument has been successfully tried out on racing cars and found perfectly reliable under hard conditions.

Incidentally, care is required when changing the fuel system to ensure that the carburettor is suited to the mode of supply. If the float system has been suitable for gravity feed, and a pressure system using a head of some 5 or 6ft. of petrol, flooding may occur due to the pressure of petrol on the needle being greater than that exerted by the float to close it. This means that a modified needle or seating which gives a smaller area of needle exposed to the fuel, and consequently a smaller thrust on it, must be fitted. An example is the U-type Zenith in which the actual seating is easily removed

and a pressure type substituted. The pressure seating is merely of smaller diameter.

Another type of pump is that which is mechanically operated by the engine, of which the A.C. Sphinx product is an example. A plunger operating a diaphragm is driven from an eccentric, driven in turn from the engine. The fuel feed is automatically regulated to the supply required, and the pressure is controlled between fixed limits to avoid flooding. The pump is, however, of the full pressure type, and the carburettor must be arranged to suit. mar A mechanical fuel pump is also

keted by the well known accessory firm of Cooper-Stewarts. This is designed to be mounted on the crankcase and driven at half engine speed from the camshaft. The diaphragm actuating mechanism is extremely simple and the down-stroke of the operating lever is cushioned by a leather bumper, which makes for quiet operation and minimum wear.

The well-known Autopulse is another example of the electric fuel pump, and has been used with success on many makes. There is now a general tendency to abandon the old methods of fuel feed in favour of the neater and simpler systems now available.

The fuel pump is, of course, the main item in the system, and reliability can only be assured by attention to details. The filtering of the fuel on leaving the tank is of the greatest importance, and if your car is not already fitted with a good filter one should be obtained as soon as possible. niters are of various types, the wire gauze being the commonest, while chamois leather is also used. A very ingenious and effective type is the Zenith plate filter, in which a large number of slightly indented plates are pressed together, and the fuel filters through the fine gaps between them. Owing to its construction, there is a very large area of filter surface and therefore no chance of starvation, while water is separated from the petrol and falls to the bottom of the bowl. The provision of ample area is one of the most important points of filter design, as in a fast car there are times when a consider

able flow of fuel is required, and in the case of a gauze filter of small size, a small amount of dirt on the mesh will seriously restrict the flow. There are many amateurs who have to instal their own fuel systems, as when building up a hill-climb ” special” of some sort, and the pipe line must be carefully arranged to avoid breakage. Long unsupported sections must be avoided, and as all chassis are subject to a certain amount of distortion it is a good thing to cut the pipe between chassis and engine, and insert a flexible connection. Special flexible petroltubing can be ob

tained, and is very useful for such work, but its cost is fairly high, and for piping on the chassis, would seem unnecessary. The copper pipe should be carefully annealed by heating to a dull red and quenching in cold water.

Special tanks are also required at times for such installations, and if they cannot be improvised from some existing tank, it is as well to get one made by some firm such as the Gallay Radiator Company, who specialise in such work.

Care and thought spent on the methods of transferring fuel from the tank to the carburettor are well repaid, and the present time is very suitable for looking over such details before next season.