A carburetor is a metering device whose function is to mechanically blend liquid fuel with a certain amount of air to produce as nearly a homogeneous mixture as possible, and in such proportions as will result in as perfect an explosive mixture as can be obtained.

If a gas is used as a fuel it is of course not so difficult to obtain a homogeneous mixture due to the intimacy with which a gas will mechanically mix with air. This intimacy is a result of the minuteness of the molecules of both the gas and the air. With a liquid fuel, such as gasoline, however, it is quite different, especially with low test gasoline. If it were possible to completely transfer the liquid fuel into its vapor the latter would act as a gas and would, therefore, mix with the air to form a homogeneous mixture. It should be, and is, therefore, the aim of the carburetor designer to produce an instrument which will atomize the fuel and break it up into small particles so that every minute particle of the fuel will be surrounded by a correct proportion of air when it is discharged into the intake manifold of the motor. To facilitate the vaporization of these minute particles of fuel it is advisable to preheat the air taken into the carburetor, thereby furnishing the necessary heat units required to vaporize the fuel by virtue of its physical property known as its latent heat of evaporation.

There is a range of proportions of air to vapor, for a given fuel, between which combustion will obtain. This range extends from that proportion known as the upper limit of combustion to that known as the lower limit of combustion. The upper limit is reached when the ratio of air to vapor is a maximum at which combustion will take place; that is to say, any addition of air in excess of this maximum will render the mixture non-combustible. The lower limit is reached when the ratio of air to vapor is a minimum at which combustion will take place, any decrease of air below this minimum producing a non-combustible mixture. It should be remembered that the limits of combustion of any fuel with air are dependent upon the temperature and pressure.

Under given temperature and pressure the rate at which the combustible mixture will burn depends upon the ratio of air to vapor. This rate of burning is known as the rate of propagation, and it is apparent that it is desirable to obtain a mixture whose rate of propagation is a maximum, because the force of the explosion will depend upon the rapidity with which the entire mixture is completely ignited.

The limits of combustion of gasoline (.70 sp. gr.) can be taken approximately as follows: lower limit, 7 parts air (by weight) to 1 part gasoline, upper limit, 20 parts air to 1 part gasoline.

The Stromberg Plain Tube Model M Carburetor.—A plain tube carburetor is one in which both the air and the gasoline openings are fixed in size, and in which the gasoline is metered automatically, without the aid of moving parts by the suction of air velocity past the jets.

Fig. 26 shows a longitudinal section of a type M plain tube carburetor, and shows the location of the gasoline when the motor is at rest. The various parts are indicated by names and arrows. An elementary requirement of a carburetor is that as a metering device it shall properly proportion the gasoline and air throughout the entire operating range.

In the carburetor under discussion this mixture proportioning is properly maintained by the use of what is termed the air bleed jet. Fig. 27 shows the principle of the action of the air bleeder. The gasoline leaves the float chamber, passes the point of the high speed adjusting needle, and rises through a vertical channel “B.” Air is taken in through the air bleeder “C,” and discharged into the gasoline channel before the latter reaches the jet holes in the small venturi tube “E.” The result is that the air thus taken in breaks up the flow of gasoline and produces a finely divided emulsion. Upon reaching the jet holes of the small venturi tube this emulsion is discharged into the high velocity air stream in the form of a finely divided mist. If the reader will recall how thoroughly a soap bubble divides itself when it bursts, he will readily appreciate how completely the air bleed jet will atomize the fuel.

Before explaining the operation of the accelerating well it is advisable to know the reason for its existence. Suppose we had a large tube such as the intake manifold of a motor through which air and particles of gasoline were flowing due to a certain suction at one end. What would be the result if we suddenly increased the suction? It would be this: Due to the fact that air is so much lighter than gasoline, the air would respond almost instantly to the increased suction and its flow would be accelerated very suddenly, whereas the particles of gasoline, owing to that characteristic known as inertia, would not respond so rapidly, and due to its heavier weight its flow would not accelerate as much as the air. This would mean that the air would rush ahead of the gasoline particles, and the proportion of air to gasoline would be greater until the inertia forces had been overcome and the gasoline particles responded completely to the increased suction. This very thing will take place in a carburetor unless provision is made for it. That is to say a sudden opening of the throttle will tend toward producing a very lean mixture at the motor due to the lagging of the gasoline explained above. A lean mixture at this time, when acceleration is desired, would obviously be detrimental to the result wanted. It is at this particular time that additional gasoline is most desirable in order to compensate for the lagging gasoline and maintain the proper mixture at the motor. In the Stromberg carburetor this is accomplished by means of the accelerating well shown in Fig. 28. The operation is as follows: The action is based upon the principle of the ordinary U tube. If a U tube contains a liquid, and if pressure is applied to one arm of the tube, or what is the same, if suction is applied to the other arm, it is self-evident that the level of the liquid will rise in the arm on which the suction is applied and will drop in the other arm. So it is in the construction of the accelerating well. Referring to the illustration, Fig. 28, the space “F” forms the one arm of the U tube, and the space “B” the other arm. These spaces communicate with each other through the holes “G” thus forming a modified form of U tube.

When the motor is idling or retarding in speed, the accelerating well or space “F” fills with gasoline. Now when the throttle is opened, thereby increasing the suction in the venturi tube, the following takes place: atmospheric pressure at the bleeder “C” exerts itself on the gasoline in the space “F” forcing the liquid down to join the regular flow from “H” and passing up the space “B” and out into the high velocity air stream through the small venturi tube. While the well acts the flow of gasoline is more than double the normal rate of flow, thereby compensating for the lagging of the gasoline referred to previously.

Upon close observation it will be noticed that there is a series of small holes down the wall of the well. Referring to the analogy of the U tube, these holes directly connect the two arms of the U tube. It is obvious that the smaller and fewer these holes, the faster will the well empty, due to the U tube suction, and the larger and more these holes, the slower will the well empty. It is therefore apparent that the rate of discharge of the well can be regulated as required by different motors, different grades of gasoline, different altitudes, etc., by inserting wells of different drillings. The action of the well is also dependent upon the size of the hole in the bleeder “C” because it is the relative area of this hole in the bleeder as compared to the area of the holes in the well which determine the rate at which the well will empty.

The foregoing characteristics of the model M carburetor have dealt more with the open throttle or high speed operation. We shall now consider the operation when the motor is idling. Earlier types of carburetors, when high test and very volatile gasoline was employed, were designed with a mixing chamber in which the gasoline, after being discharged from the nozzle, would mix with the air and evaporate very freely. Present day gasoline, however, is considerably heavier and very much less volatile, and we therefore cannot depend upon its volatility to accomplish its vaporization.

Fig. 29 shows the arrangement and idling operation of the model M Stromberg carburetor. Concentric and inside of the passage “B” is located the idling tube “J.” When the motor is idling, that is, when the throttle is practically closed, the action which takes place is as follows: the gasoline leaves the float chamber, passes through the passage “H” into the idling tube through the hole “I,” thence up through the idling tube “J” to the idling jet “L.” Air is drawn through the hole “K” and mixes with the gasoline to form a finely divided emulsion which passes on to the jet “L.” It will be noted that this jet directs the gasoline-air emulsion into the manifold just above the lip of the throttle valve. Inasmuch as this throttle valve is practically closed the vacuum created at the entrance of the jet “L” is very high and exceeds 8 pounds per square inch. It is obvious, therefore, with this condition existing, that the gasoline will be drawn into the manifold in a highly atomized condition. It is well to call attention here to the fact that the low speed adjusting screw “F” operates a needle valve which controls the amount of air which passes through the hole “K,” and it is the position of this needle valve which determines the idling mixture.

As the throttle is slightly opened from the idling position a suction is created in the throat of the small venturi tube as well as at the idling jet. When idling the suction is greater at the idling jet, and when the throttle is open the suction is greater at the small venturi tube. At some intermediate position of the throttle there is a time when the suction at the idling jet is equal to that at the small venturi, and, therefore, at this particular time the gasoline will follow both channels to the manifold. This condition which is illustrated in Fig. 30 lasts but a very short while, because as the throttle is opened wider the suction at the small venturi tube rapidly becomes greater than that at the idling jet. The result is that the idling tube and idling jet are thrown entirely out of action, the level of the gasoline in the idling tube dropping as illustrated in Fig. 31, where the throttle is shown to be wide open, in which case all of the gasoline enters the manifold by way of the holes in the small venturi tube.

It will be remembered that at this position of the throttle the accelerating well has emptied, and therefore there is a direct passage for air from the bleeder to the gasoline in the main passage giving the air bleed jet feature explained before. This is being mentioned again in order to call attention to the fact that care should be taken not to use too large a bleeder, because the air which enters through the bleeder partly determines the mixture, and if the bleeder hole is too large the mixture is very apt to be too lean at high speeds.

Fig. 32 shows an exterior photograph of one of the type M Stromberg carburetor. Before discussing the installation and adjusting of this carburetor it will be well to say a few words concerning the use of the venturi tube and its construction.

The object in using the venturi tube in carburetor design is to produce a maximum air velocity at the jet and at the same time not cause undue restriction. This high air velocity creates the suction necessary to properly atomize the gasoline. The use of the double venturi tube construction has developed the best possible results. In this construction the mouth of the smaller venturi tube is located at the throat of the larger one, and with this arrangement the highest degree of atomization is attainable, and at the same time the air restriction is held down to a minimum.

In order that any carburetor may do justice to what is claimed for it, it is absolutely essential that the motor on which it is installed is in good condition in other respects because, besides poor carburetion, there are numerous things about an internal combustion engine which will cause its poor operation. Therefore, assuming that the following conditions exist, we can proceed with the installation of the carburetor and after adjusting it we can expect very good results as to the operation of the motor.

1. The ignition should be properly timed so that with a retarded spark the explosion takes place when the piston of the cylinder in which the explosion occurs is at its upper dead center.

2. The inlet and exhaust valves should be so timed that they open and close at the proper time during the cycle. In this respect a motor is usually timed when it comes from the manufacturer.

3. The valves should be ground in so that they form a perfect seal with the valve seat. Any accumulation of carbon on the upper part of the exhaust should be removed so as to prevent the valve stem from sticking in the guide and thereby not permitting the valve to close upon its seat.

4. Any undue wear of the valve stem guides should be corrected because the clearance between the stem and the walls of the guide will permit air to be drawn up into the motor thus ruining the mixture from the carburetor. Similarly any leaky flange at any joint along the intake system will produce the same detrimental result.

5. All piston rings should be tight and leak proof in order to insure good and even compression in all the cylinders. Without good and even compression in all the cylinders it is impossible to obtain the maximum power from the motor, and it is also impossible to obtain good idling of the motor.

6. It should be seen that the ignition system is delivering a spark to each spark plug without missing.

7. The spark plugs should be clean, and the accumulation of carbon on the inside of the plug should not be sufficient to cause fouling or short-circuiting of the plug. In the case of a short circuited plug it is impossible to obtain a spark at the end of the high tension cable, but this does not indicate that the plug is firing. For best results the gap of the spark plug should never be less than .020″ nor more than .032″. A good setting is at about .025″.

The foregoing constitute some of the more important troubles to look for when the motor is not performing satisfactorily.

Installation and Adjusting.—We are finally ready to proceed with instructions for installing and adjusting Model M carburetors.

1. Try the throttle lever and the air horn lever by moving same with the hand before the carburetor is installed, and be sure that the butterfly valves are open to the limit when the respective levers come in contact with their stops. Also be sure that when the throttle valve is closed, the lower side of the butterfly is adjacent to the hole through which the idling jet projects.

2. Prepare a paper gasket about .020″ thick to fit the flange of the carburetor. Shellac same and then attach the carburetor to the flange of the intake manifold very securely by means of proper cap screws.

The attaching of the gasoline line, hot-air stove, hot air flexible tubing, and choke control need not be discussed in detail as these installations are very simple.

After having properly installed the carburetor on the motor, turn both the high and low speed adjusting screws, A and B, completely down clockwise so that the needle valves just touch their respective seats. Then unscrew (anti-clockwise) the high speed adjusting screw A about three turns off the seat, and turn the low speed adjusting screw B anti-clockwise about one and one-half turns off the seat. These settings are not to be considered as final adjustments of the carburetor. They are merely to be taken as starting points because the motor will run freely with these settings.

After the motor has been started, permit it to run long enough to become thoroughly warm then make the high speed adjustment. Advance the spark to the position for normal running. Advance the gas throttle until the motor is running at approximately 750 r. p. m. Then turn down on the high speed screw A gradually notch by notch until a slowing down of the motor is observed. Then turn up or open the screw anti-clockwise until the motor runs at the highest rate of speed for that particular setting of the throttle.

To make the low speed adjustment B proceed as follows: Retard the spark fully and close the throttle as far as possible without causing the motor to come to a stop. If upon idling the motor tends to roll or load it is an indication that the mixture is too rich and therefore the low speed screw B should be turned away from the seat anti-clockwise, thereby permitting more air to enter into the idling mixture. It is safe to say that the best idling results will be obtained when the screw B is not much more or less than one and one-half turns off the seat.

After satisfactory adjustments have been made with the motor vehicle stationary, it is most important and advisable to take the vehicle out on the road for further observation and finer adjustments. If upon rather sudden opening of the throttle the motor backfires, it is an indication that the high speed mixture is too lean, and in this case the high speed screw A should be opened one notch at a time until the tendency to backfire ceases. On the other hand if when running along with throttle open, the motor rolls or loads, it is an indication that the mixture is too rich, and this condition is overcome by turning the high speed screw A down (clockwise) until this loading is eliminated.

STROMBERG MODEL L CARBURETOR

There are three adjustments; the high speed, the extremely low speed or idle, and the “economizer.”

The high speed is controlled by the knurled nut “A” which locates the position of the needle “E” past whose point is taken all the gasoline at all speeds. Turning nut “A” to the right (clockwise) raises the needle “E” and gives more gasoline, to the left, or anticlockwise, less.

If an entirely new adjustment is necessary, use the following practice. Put economizer “L” in the 5th notch (or farthest from float chamber) as an indicator, turn nut “A” to the left, anticlockwise, until needle “E” reaches its seat, as shown by nut “A” not moving when throttle is opened and closed. When needle “E” is in its seat it can be felt to stick slightly when nut “A” is lifted with the fingers. Find adjustment of “A” where it just begins to move with the throttle opening, then give 24 notches to the right or clockwise (the notches can be felt). Then move the economizer pointer “L” back to the 0 notch (toward float chamber). This will give a rich adjustment. After starting and warming up the motor, thin out the mixture by turning “A” anticlockwise, and find the point where the motor responds best to quick opening of the throttle, and shows the best power.

The gasoline for low speed is taken in above the throttle through a jet at “K” and is regulated by dilution with air as controlled by the low speed adjusting screw “B.” Screwing “B” in clockwise gives more gasoline; outward, less. The best adjustment is usually ¹⁄₂ to 3 turns outward from a seating position. Note that this is only an idling adjustment and does not effect the mixture above 8 miles per hour. When motor is idling properly there should be a steady hiss in the carburetor; if there is a weak cylinder or manifold leak, or if the idle adjustment is very much too rich, the hiss will be unsteady.

The economizer device operates to lean out the mixture by lowering the high speed needle “E” and nut “A” a slight but definitely regulated amount at throttle positions corresponding to speeds from 5 to 40 miles per hour. The amount of drop and consequent leaning is regulated by the pointer “L.”

After making the high speed adjustment for best power, with pointer “L” in 0 notch, as above described, place throttle lever on steering wheel to a position giving about 20 miles per hour road speed. Then move pointer “L” clockwise (away from float chamber), one notch at a time, till motor begins to slow down. Then come back one notch.

The amount of economizer action needed depends upon the grade of gasoline and upon the temperature.

In the mid-west the best economizer adjustment will usually be the third or fourth notch. With Pennsylvania gasoline and in the South, the 2nd notch; while on the Pacific coast no economizer is necessary unless distillate (which should not be below 59 degrees Baume) is used. Also fewer notches economizer action will be necessary in summer than in winter.