TUNING CARBURETTORS FOR COMPETITION EVENTS.

TUNING CARBURETTORS FOR COMPETITION EVENTS. The ” Solex ” Carburettor.

THE ” Solex ” carburettor occupies a deservedly popular place as the standard equipment for a large number of different makes of sports cars, and has achieved many notable successes in international racing events. Perhaps its most prominent feature is the remarkable degree of accessibility the construction affords, which in conjunction with. the ease of tuning makes it an ideal instrument not only for the everyday motorist, but also for competition work, where such a simple thing as a choked jet may easily mean the loss of marks, or a bronze award instead of a gold.

No doubt many of our readers are vaguely familiar with the principles under which the Solex carburettor operates, but for the benefit of those who may know little of the theories involved we will give a brief descript tion, which can be followed readily by reference to the sectional illustration appearing below.

How the ” Solex ” Carburettor Works.

The petrol is fed in to the top of the float chamber 0, through the union P, which is secured in position by the part Q, this being a combined set screw and body for the petrol filter, located within the union. The petrol then passes through the needle valve seating and fills the float chamber to the required level, when the rising float l? comes into contact with the lower end of the needle valve P, and closes it on to its seating, thereby regulating the level. A flooder T, with a spring return, is fitted in the top of the float chamber to facilitate starting, if necessary.

The setting of the needle valve is arranged to shut off the petrol supply slightly below the level of the pilot, or auxiliary jet, G, from which a passage cast integrally with the body of the carburettor leads to the engine intake and is arranged to supply the fuel for slow running and idling.

In the more recent types of Solex carburettor, the original barrel throttle has been superseded by the butterfly type, as shown in the illustration, thus giving a more definite shut-off and other advantages that were not so marked in the earlier form. K is the choke tube, which is held in position by the set screw U, and is readily removable during the tuning operations. The jet assembly A is mounted in the lower portion of the carburettor, the” correction” with which our readers are now quite familiar being obtained as follows :—

The main submerged jet G is secured in a petrol tight manner at both ends by the outer cap A, which screws down upon the union T, the lower end of the jet being calibrated in accordance with the requirements of the engine, and, in addition, two holes are provided at the lower end of the jet. Between the main jet and the outer cap is a sleeve, the height of which extends a little above that of the level of the petrol, as controlled by the float chamber needle valve. In the outer cap A there are also two holes in the bottom and one in the centre at the top, the bottom ones being well below the choke tube, and are therefore under atmospheric pressure alone, whilst the upper one is surrounded by the choke tube, and thus falls under the influence of the engine suction.

When air is drawn tht ough the choke tube K, by the engine, air is also drawn in through the lower holes of the outer cap A, and passes up and over the top of the intermediate sleeve. From thence it passes down between the sleeve and the main jet, and passing through the holes in the bottom of the main jet G mixes with the fuel passing into the engine via the top of the centre hole. The greater the suction of the engine, the more air passes through the respective holes of the outer cap and the bottom of the main jet, the suction on the submerged jet orifice being reduced accordingly, and in

this way a correct balance of fuel to air is provided at any engine speed. The dimensions of these holes are of the greatest importance to the proper working of the carburettor, and having once been calibrated by the manufacturers, should on no account be altered.

Adjustment for Slow Running. When the is with the throttle

When the throttle closed, the supply of fuel is drawn entirely through the pilot, or auxiliary jet G, the number stamped on this jet indicating its size and refers to the diameter of the orifice in hundreds of millimetres. As the jet orifices are calibrated by the manufacturers by the aid of a flowmeter, no attempt should be made to effect any alteration either by reamering or closing, but all alterations should be made by selecting a suitable pilot jet to suit the requirements of the engine.

The adjustment for slow running is regulated independently and with no reference whatever to the conditions of the main setting, as the latter is entirely inoperative at low throttle positions. The following conditions regulate the efficiency of the slow running :— 1, Regulating the slow running mixture ; and 2, Regulating the minimum engine speed.

Regulating the Slow Running Mixture.

This is done by selecting a jet that will enable the engine to idle without any of the following symptoms :-

Excess of Fuel.

The use of an excessively large pilot jet will cause the engine to run with an irregular rhythm, commonly known as “hunting,” and in such a case, after running slowly for a short time and switching off, a quantity of fuel will drip from the carburettor on opening the throttle. Should this occur, a pilot jet of smaller dimensions should be inserted and the slow running again tested.

Insufficient Fuel.

This is indicated by an irregularity in the slow ninning, but no particular rythm can be detected. If the jet is too small, the slow running will be improved temporarily by depressing the floodder T. If, also, the auxiliary jet is too small, considerable difficulty will be experienced in starting the engine. With regard to the latter point, it is usually found desirable to use an auxiliary jet of about one size larger than that which gives the leanest slow running mixture, in order to ensure easy starting, especially in cold weather. But, on the other hand the accessibility of the carburettor is so great that the changing of the jets for any conditions can be effected with the greatest of ease by anyone who really understands the principles under which the carburettor operates, though, of course, the practice of indiscriminate tinkering should be rigidly banned.

The Regulation of Minimum Engine Speed.

Assuming that the selection of the auxiliary jet has been made in accordance with the above instructions, all that remains to be done with regard to this part of the operation is that of fixing the minimum engine speed, which is effected by adjusting a screw (not shown on the section), and this controls the amount by which the throttle is closed, and when once determined, this setting can be secured by a locknut, so that no further adjustment will be necessary.

Adjustment for Power.

The first point in tuning the main part of the carburettor is the selection of the choke tube, and as a rule the information as to size, provided by the makers, will be found correct with regard to different types of engines. Each choke tube has a number stamped on the inside, and this refers to the internal waist measurement in millimetres. When an approximately correct size of choke tube has been selected, the adjustment for power now becomes a matter of ascertaining the right

size of the main jet 0, the Solex system being so simple that these two adjustments alone suffice to give the best results, the intermediate sleeve and the outer cap remaining constant for various sizes of main jet. Thanks to the general accessibility of the carburettor, nothing could be simpler than changing the main jet, but attention is again directed to the avoidance of alteration of jet orifice sizes by reamering, or otherwise.

Generally speaking, the best results will be obtained by selecting the smallest size of main jet possible, consistent with good running power. If the main jet is too small, the engine will evince a tendency to hesitate on acceleration, which will be accompanied by the usual “popping” in the carburettor when cold. If, on the other hand, the size of jet selected permits the engine to answer to the throttle instantly on full opening, it may be taken that a smaller size of main jet can be used with advantage. In tuning the Solex carburettor, as with all others, the selections should be made after the engine has been allowed to warm up.

Testing the Petrol Level.

The arrangement whereby the two main portions of the Solex carburettor are secured by the nut E is very convenient when it is desired to test the petrol level. In this case the lower portion is released and swung round, so that the jet stand is clear of the choke tube, and then when the petrol is again turned on it is possible to see whether the needle valve shuts off the supply at the correct height with regard to the jets, or otherwise. The adjustment of level can be made by inserting thin washers beneath the needle valve seating at J2, or in certain cases by reversing the float, which will have the effect of raising the level.

The Solex carburettor is supplied either in the horizontal or vertical type, the same degree of accessibility being equally notable in both patterns. Very good workmanship and a careful calibration of all essential dimensions enables the user to obtain the best results for any given class of engine. The manufacturers are Messrs. Solex Ltd., of 222, Marylebone Road, N.W.1.