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The Why of Carburetter Adjustments.

11th April 1912, Page 4
11th April 1912
Page 4
Page 5
Page 4, 11th April 1912 — The Why of Carburetter Adjustments.
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Which of the following most accurately describes the problem?

Modifications for High, Low arid Medium Speeds and for Load.

By an EngineerCorrespondent.—, oncluded from Page of

If there is a constant dripping of petrol when the engine is not running, the float-operated needle valve should be examined, and if by pressing it down on to its seat the leaking ceases, the float evidently is too high on its spindle ; should the dripping continue in spite of the finger,. the valve requires regrinding on to its seat, and for that purpose the use of pumicestone is recommended in preference to emery flour, because small particles of the latter material become embedded in the brass seating, and trouble quickly follows. When grinding in a float valve, care must be taken that it is kept perpendicular to the seat, or the result will be that, although the valve may be tight when the float is at one particular level, as soon as the level is altered, as a result of subsequent adjustments of the float, the spindle is canted over, and a leak shows at the valve seat. If the spindle be provided with top and bottom guides, canting is not very likely to occur, hut it is always wise to take precautions to insure that a good seating is obtained for all levels of the float.

Adjustment for Low Speed.

In automatic carburetters, the auxiliary-air valve is not intended to operate when the engine is running at low speeds ; consequently, for such speeds, one has only to deal with the size of the jet, or of the area of the main-air inlet. In some carburetters, the mainair inlet is non-adjustable, but the size of the jet may be varied, whilst in others the jet cannot be altered, but the air inlet may be adjusted. A reduction in the size of the main-air inlet is equivalent to an increase in the size of the hole in the jet. The adjustment of ttIl engine to run at low speeds, therefore, is a fairly easy matter.

Let us now assume that the engine has been started, and is running smoothly at a low speed, with the spark retarded, and the throttle valve partly open. If, now, the throttle valve be opened further, the engine should accelerate smartly, and its maximum speed (when the change-speed gears are in neutral position) should be attained when the throttle-valve lever has been moved through only about one-eighth of its possible movement. The auxiliary-air valve should open, but not by any means to its full extent, as the engine is only taking in a small charge of gas at each piston-stroke. If the auxiliary-air valve opens fully under the conditions named, the spring which regulates it is evitiently too weak, and it should be replaced by a stronger one, or, if it is adjustable, its tension or compression should be increased. Adjustment for Load.

'the next step in the adjustment of the carburetter is to try the vehicle at moderate speeds on the road, and it is here that the real business of carburetter adjustment begins. Let us suppose that the engine is sluggish. Without stopping it, the effect of slight changes in the jet opening should be tried, or it the size of the jet is non-adjustable, by varying the mainair opening. It should not be forgotten that, by reducing the size of the main-air inlet, the mixture is enriched, and, conversely, byincreasing the opening, the percentage of petrol in the mixture is decreased. First try enriching, and then impoverishing the mixture, and note Eke effect upon the performance of the vehicle when it is running on the level. Although a position of adjustment at which the engine runs well at, ordinary speeds may be found, one cannot be sure that that mixture is the best one. It is possible that he engine is being driven with the throttle valve too wide open. It should be noted if the radiator heats up while running on the level ; also, if the engine shows signs of weakness and overheating when hillclimbing. These symptoms sometimes indicate too rich a mixture.

The working of the auxiliary-air valve must next be watched. It should open from in. to in. according to its size, but it is difficult to give a definite figure, because so much depends upon of the carburetter. If the mixture is too rich, the auxiliary air valve must be so adjusted that more air is admitted. This may he done by slackening the auxiliary-valve spring, but it should be remembered that any change in the spring's tension has a greater proportional effect at low speeds than at high speeds. If easing the spring helps matters at medium speeds, but not at high speeds, it should be observed if the auxiliary-air valve opens to its greatest extent before the engine speed reaches its maximum ; if it does, the high-speed mixture is too rich, and, if the construction of the auxiliary valve will allow of its being given a wider meaning, this should be done. If. however, the s :Live flutters, and works irregularly, the lift must be re

stricted by increasing the tension of the spring, at the same time reducing the size of the petrol jet. If the size of the jet opening cannot be altered, the main-air opening should be closed a little, in order to allow of a larger quantity of air entering through the auxiliaryair valve. Such an adjustment has the effect of

weakening the mixture more at high speeds time at low speeds, which is exactly what is required.

If, instead of overheating, the motor simply shows signs of weakness, it may safely be assumed that the mixture is too much impoverished as the result of a weak auxiliary-valve spring, which spring allows the valve to open fully, even when the engine is running at low speeds.

Adjustmentfor High Speed.

It is not very difficult to modify the low-speed adjustment so that the engine will give its maximum power and speed, and this should be the next step. On this point, there is less room for uncertainty than with the low-Speed adjustment, because, unless the proportions of the mixture are nearly correct, the maximum car speed cannot be reached. A very good test of the quality of the mixture is the way in which the engine responds to the spark-advance lever—if one be fitted. lithe mixture is bad, the spark must be advanced through a considerable angle before there is any appreciable effect on the engine's speed. On the other hand, if the mixture is nearly correct, the slightest movement of the lever is immediately followed by an alteration in the engine's speed.

Correct Adjustment for Medium Speeds.

When the high-speed adjustment has been found, the driver must then ascertain whether the car will run well at medium speeds. If it will not do so, he should first think over what he has done in the way of adjustment for the low speed, and also for the high speeds. For instance, if, in order to get the engine to run well at high speeds, he had found it necessary to increase the tension of the auxiliary-air-valve spring, which adjustment had the effect of enriching the mixture by causing more air to pass the jet, it will be realized that such aa adjustment would have the effect of giving a still-richer mixture all the lower speed. Such a condition as is here assumed indicates that the auxiliary-valve spring is made of an unsuitable size of wire, or that in has too many coils, and, in consequence, has a very low " rate" of deflection ; a very slight pressure will cause it to compress a relatively large amount. With such a spring, it is probable that it would have to he subjected to such a heavy initial compression, in order to prevent the valve's opening at low speeds, that it would not yield until a fairlyhigh engine speed is attained. Before discarding the

spring, however, slackening of the tension again should be resorted to, sufficiently to allow the valve to open when the engine is run at moderate speeds; the necessary counter-acting adjustment to the carburetter, either by increasing the size of the jet, or by slightly reducing the main-air opening—either of which operations has the same effect as tightening up the auxiliary springs—must be considered.

If the previous adjustment of the jet and the air opening should have proved satisfactory for both lowspeed and high-speed running, it should be carefully marked before being corrected for medium speeds, in order that it may be re-adjusted to the satisfactory position in case the modifications in adjustment for medium speeds should prove ineffectual. It has been assumed that at moderate speeds the engine did not run well because the mixture was too rich ; if, however, tho mixture was too weak, it is obvious that the auxiliary-valve spring must be tightened a little, but, in order that the high-speed adjustment may not be upset thereby, the jet should be slightly reduced.

Don't Adjust Thoughtlessly or Hurriedly.

A. driver should never be in a hurry about making adjustments: one thing should be tried at once, and tried thoroughly, and the driver should always bear in mind that whatever alteration is made in the tension of the auxiliary-valve spring, that change will have the greatest effect on the running of the engine st its moderate speeds. and the least effect at high speeds. If, after experiments, it is impossible to find it state of adjustment under which the engine will run well, it may safely be assumed that the size of the carburetter is not suitable for the engine. This article has been restricted to actual working adjustments, and, throughout, it has been assumed that the usual and necessary precautions for cleanliness and filtering have been observed.

The foregoing notes are only intended for those drivers and mechanics who really are capable of following out the instructions given ; the steersman pure and simple should " let well alone." Leave the carburetter alone, therefore, is the earnest warning to every driver who does not feel competent to digest the foregoing notes.


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