Principle of Operation; The Compensating Jet and Compound Nozzle; Theory Applied in Zenith Practice; Installing, Adjusting and Checking a Zenith Carburetor; Carburetor Adjustments; Servicing and Maintenance of the Zenith; Zenith Model SV; Care of the Carburetor; Factors Which Assist Good Carburetion.
The Zenith Carburetor is a plain-tube type of carburetor with fixed adjustments.
In order to make clear the principle' of this carburetor, simplified illustrations and explanation are given below taken from the Zenith pamphlet en-titled "The Balanced Ration."
The Balanced Ration
Just as the food we eat must contain the right proportions of carbohydrates, proteins, fats, etc., in order to keep the body working at its highest efficiency, so, the automobile engine must be fed exactly the right proportions of gasoline and air, in order that it may function properly.
In each case, the highest pitch of efficiency—the Zenith —is reached only by means of a perfectly balanced ration. Appetites may vary, greater exertion of either the human body or the automobile engine will call for a larger ration; but always the ration must be balanced, must contain the same kinds of foods in the same proportions in order to produce the best results.
Few of us devote very much real thought to the subject of diet. We prefer to leave the matter to those in charge of the kitchen, whose particular task it is to see that we are supplied with the various foods in the proper quantities to form the balanced ration necessary to sustain our energies.
The engine too is dependent upon its kitchen. Its cylinders take in and digest the food and turn it into energy just as the human stomach does, but before the food reaches the cylinders it must be measured out, the proper proportions of fuel and air must be brought together and mixed thoroughly and so prepared for the engine's consumption. The de-vice that performs this function—the link between the raw food and the prepared meal—the kitchen, in fact, which supplies the engine with its balanced ration—is the carburetor.
The Ordinary Ration
The simple carburetor: A simple carburetor is one having a fuel chamber, a single air entrance and a single jet (see Fig. 1). Suction, created by the pumping of the pistons, causes fuel and air to flow through the carburetor into the engine. Each alternate downward stroke of a piston draws a fresh charge of mixed fuel and air from the carburetor into its own particular cylinder, where it is compressed and exploded.
The simple carburetor won't do. However, fuel is more responsive to suction than is air. Consequently as the engine gathers speed the flow of fuel into the engine ihereases much faster than the flow of air, the mixturebecoming too rich. It is no longer the perfectly balafced `ration which the engine needs in order to do its best work.
Attempts to overcome the defects: More speed should be a matter of a larger ration, not a richer ration. Many attempts have been made to over-come this natural tendency of the mixture supplied by the simple carburetor to become rich. However, due to mechanical difficulties or sensitiveness to changeable atmospheric and temperature conditions many of these devices have proven unsatisfactory.
How the defects were overcome: The French scientist, M. Baverey, inventor of the Zenith carburetor, solved the problem of the "balanced ration" by taking the direct and natural method of so arranging the fuel jets that they accurately proportion the fuel flow to the air flow in accordance with the suction. He used natural methods, not mechanical methods.
To overcome the variation of the simple jet which allows the mixture to grow richer under increasing suction, Baverey introduced another, calling it the compensating jet, which has exactly the opposite effect, allowing the mixture to grow "poorer" under increasing suction. He then combined the two jets into one—the compound nozzle—and achieved the desired result, a carburetor which delivers at all engine speeds a mixture containing exactly the right proportions of air and fuel—the perfectly balanced ration.
How Zenith Balances the Ration
In Zenith carburetors there are four measuring parts, supplied to meet the exact requirements of any particular engine:
1. The choke tube (X) (Fig. 4)—This measures the amount of air taken into the engine. The flow of air increases as the engine speed increases.
2. The main jet (8) (Fig. 4)—This acts exactly like the jet in the simple carburetor. It varies in flow with the suction.
3. The compensating jet (6) (Fig. 4)—The flow from this jet is constant regardless of the amount of suction, or the speed of the engine.
4. The idling jet (2) (Fig. 4)—This operates only when the throttle is barely cracked open. Further opening of the throttle automatically puts the idling device out of operation because the fuel in the well is then all drawn through the cap jet (4) (Fig. 4).

How the Main Jet Acts
Look at Fig. 1. You will see that (G), the main jet, is directly connected with the fuel chamber (F)
Compare the fuel chamber to a bottle and the main jet to a straw.
Now, if you put a straw down to the bottom of a full bottle (Fig. 1A), you will find that the harder you suck on the straw the more liquid you will get.
The suction of the engine will act on the fuel in
1 ltep-inted from Zenith instruction booklets. the bowl through the main jet the same as your
see also pages 130, 131 of Dyke's Auto Encyclopedia section on the straw acted on the liquid in the bottle.

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Carburetor Manuals: Zenith