Minor Peculiarities Of the Modern Carburettor

Poverty Bay Herald, Volume LXVI, Issue 19885, 11 March 1939, Page 11

Minor Peculiarities Of the Modern Carburettor

| JT is apparent that the minor peculiarities of the modern downdraught, carburettor are not yet fully understood by many motorists, although the subject has received a good deal of publicity. The most common fault is difficult starting when the car has been left leaning into a kerb or facing up or down hill. The same difficulty occasionally is experienced when the engine is rather hot. This is caused by an excess of fuel in the induction manifold, because of certain features of the downdraught design, which otherwise is the best type of carburettor the manufacturers have yet provided. Also, once proper method of starting is understood, this one difficulty disappears. It is only necessary to hold the throttle wide open and operate the starter until the engine fires, which it will do without delay. Unfortunately, a number of motorists use the old trick of pumping at

the throttle. This is fatal, as the modern carburettor has an accelerating pump directly connected with the throttle arm, designed to spray a measured amount of excess fuel into the induction manifold to give better acceleration. Therefore, every time the throttle is pumped with the

foot, this extra quantity of fuel goes ino the manifold until it is swamped, and starting then becomes impossible until some of it has evaporated. Carburettor Adjustments A number of motorists have also been guilty of experimenting with carburettor settings, having little knowledge of the theory of the subject, and only achieving such disproportionate metering of the fuel at various speeds, that the engine develops a horrible “flat spot.” The unwisdom of interfering with the factory settings cannot be too strongly emphasised. The fixed jets have been worked out very carefully as to size to provide a good balance between power output and fuel consumption. If the owner suspects, however, that they do give a slightly too rich mixture, which is possible, as some factories make the mixture too strong deliberately for the runningin period, then the car should be returned to the service station for the simple but rather accurate alterations necessary. The one carburettor adjustment the owner can make is that, relating to slow running. On the side of the carburettor barrel will be seen a little knurled screw. The turning of this to the left will weaken the idling mixture and to ,the-right will strengthen it. Experiments can be made with this until the point is reached where the engine idles evenly and will accelerate without hesitation, on the leanest possible idling mixture. Idling Speed Most, modern engines ore designed to idle at a speed equivalent to about six of seven miles an hour in top gear, and it is worth while sometimes to make on a level road the simple test of allowing the car to run along with the foot oil' the throttle to see whether the engine will do this by speedometer reading. Sometimes, this test will show that the idling speed is set too high. On other occasions, it is set so low, that the engine will stall. On the outside of the carburettor is a special means of adjustment by which the proper speed can be secured. This is part of the carburettor throttle arm and consists of a small screw and stop,

by means of which the maximum possible degree of throttle closing can be varied. If attention to this point does not produce the desired results, look to the sliding connection of the hand throttle. This may be set in such a fashion that the throttle arm is pro.' vented from going down on to its stop, holding it open more than is desirable. This also can be altered easily. When an engine idles too fast, fuel consumption is apt to suffer, as on the many occasions on country roads when the car runs down hill with the throttle closed, more fuel is : ng the cylinders than would be the case if the adjustment were correct, i