Zenith Carburettor (Baverey'sSystem) 13th Edition April 1923

The Zenith Principle Pages 2-3

In order to clearly understand the operation of your carburetor, you should become acquainted with the principle of the "Compound Nozzle" around which the Zenith is built. This principle is described in detail in our booklet: "The Compound Nozzle Explained", a copy of which will be sent on request.

Single Jet Action
To illustrate this principle briefly let us consider the elementary type of carburetor, or mixing valve as shown in Fig 1 (below) This consists of a single jet or spraying nozzle, placed in the path of the incoming air and fed from the usual float chamber. It is a natural inference to suppose that as the speed of the motor increases, both the flow of air and gasoline will increase in the same proportion.


BUT SUCH IS NOT THE CASE. What really happens is that the flow of gasoline from the jet increases under suction faster than the flow of air, giving a mixture which grows richer and richer - a mixture containing a much higher percentage of gasoline at high suction that at low. This tendency is shown by the accompanying curve, Fig 2, which gives the ratio of gasoline to air at varying speeds with this type of jet.


Auxiliary Valves
The most common method of correcting this defect is by providing various auxiliary valves, which, by adding air, tend to dilute this mixture as it gets too rich. It is well known that moving parts are affected by lack of lubrication, by dust and by air conditions. Hence it would seem unwise to run this risk of trouble if a simpler method could be employed.

Constant Flow Device
Now if we can combine with this jet which gives a flow increasing faster than the suction, a jet having a flow which does not increase, they would act to balance each other, and both together could be made to give a mixture of constant proportion. This other Jet, the compensator is shown in Fig 3 (below) Here, a certain fixed amount of gasoline determined by the opening 'I' is permitted to flow by gravity into the well 'J' which is open to the atmosphere.


The suction at jet 'H' has no effect on the flow through compensator 'I', because the suction is destroyed by the open well 'J'. As the motor suction increases, more air is drawn up through the carburettor, while the amount of gasoline remains the same, and, therefore, the mixture grows poorer and poorer. Figure 4 shows this action.


The Compound Nozzle
By combining these two types of rich and poor mixture carburettors, the Zenith Compound Nozzle was evolved. In Figure 5 (below) we have both the direct suction or richer type, leading through pipe 'E' and nozzle 'G', and the Zenith 'constant flow' device shown at 'J', 'I', 'K' and nozzle 'H'. One counteracts the defects of the other, so that from the cranking of the motor to its highest speed there is a constant ratio of air and gasoline to supply efficient combustion.


The Idling Device
In addition to the Compound Nozzle, the Zenith is equipped with a starting and idling device, terminating in a priming hole at the edge of the butterfly valve, where the suction is greatest when the valve is slightly open. The gasolene is drawn up by the suction at the priming hole, and mixed with the air rushing by the butterfly valve, giving an ideal slow speed mixture. When the butterfly valve is opened further, the idling device ceases to operate because the increased flow of air picks up the gasolene from the Main jet 'G' and Cap Jet 'H', thus cutting off the supply to the priming hole 'U' (see figure 6 below)


With the foregoing explanation, this sectional view of the Zenith Model 'L' above will make it easy to understand the 'Compound Nozzle' in its simple form, proof against wear and trouble. Special bulletins are published, illustrating and describing each different model.

Important to Zenith Users - Page 5
For the perfect working of the Zenith Carburettor it is essential that none of the parts be tampered with, or the size of the jets altered by boring, hammering, etc. The Zenith was invented in 1906, the inventor himself is actively connected with our French company, the men who helped him are still in our organization and the Zenith of today is the result of all these years of careful study and specialization by them.
That they know their business is amply testified to by the world-wide success of the Zenith. Therefore, we ask you to believe that every part of the Carburetor is well designed for its purpose and should not be changed or modified except as indicated in our printed instructions.

The jets especially should not be altered. They are carefully guaged at the factory, tested for flow of gasoline and brought to a standard size according to the number stamped on each.

If any difficulty is experienced with your Carburetor take your car to the nearest Zenith Service Station. If this is impossible, write us full details of your trouble. Always give the Model Number of the Carburetor (See letter and numeral cast on side of Carburetor, as O4, L6 etc.) the make, year and model of your car, the bore and stroke and number of cylinders of your engine. Immediate and full attention will be given and the utmost will be done to see that you get the full satisfaction and service out of your Zenith Carburetor that you have every right to expect.

Adjusting the Zenith - Pages 5-7
In the Zenith, the quality of the mixture is fixed, once for all, by the choosing of the three variables:- Choke Tube, Main Jet and Compensator, as indicated below.
The Size Number of these three parts constitute what we call the "Setting". The Size Number is stamped on the end of each part. The Chokes are numbered in millimeters according to the size of their smallest inside diameter.

The Jets and Compensators are numbered in hundredths of a millimeters. A 'one hundred' jet has one millimeters hole and is smaller than a one hundred five jet. The sizes vary regularly by steps of 5 hundredths of a millimeter.

When cars are regularly equipped at the factory with the Zenith Carburetor it is seldom necessary to change the "factory setting". These have, in every such case, been determined by experts after exhaustive tests, and as there are no moving parts in the Zenith in any way affecting the mixture, it is reasonable to assume that any trouble that might arise can be caused only by dirt and water in the Carburetor, by tampering with its setting, etc., or by the disarrangement of some adjustment of the ignition, valve operating mechanism etc. Troubles often erroneously blamed for the carburetor are described on pages 8 to 15.

The following tests should be made with method, first determining the Choke, then the Main Jet then the Compensator.

This is really an air nozzle, of such a stream line shape as to allow the maximum flow of air, without any eddies, and with the least resistance. It is held in place by a screw and can easily be changed when the Butterfly Throttle has been removed.

Choke Tube TOO LARGE
The pick-up will be defective and cannot be bettered by the use of a larger Compensator. Slow speed running will not be very smooth. The motor will have a tendency to "load-up" under a hard pull and at high speed the exhaust will be of an irregular nature. (This "loading-up" will be much worse if the manifold is too large or too cold)

Choke Tube TOO SMALL
The effect of a small Choke Tube is to prevent the motor from taking a full charge with the throttle opened fully . The pick-up will be very good, but it will not be possible to get all of the speed of which the car is capable. Bear in mind that when the Choke is increased more air is admitted and the mixture is correspondingly thinned and vice-versa.

VARIABLE 2 - MAIN JET The main jet is easily rermoved after unscrewing the Lower Plug. The influence of the main jet is mainly felt at high speed.

Main Jet Too Large
At high speed on a level road it will give the usual indications of a rich mixture; irregular running, characteristic smell from the exhaust, firing in the muffler, sooting up at the spark plugs and low mileage (low miles per gallon)

Main Jet Too Small
The mixture will be too lean at high speed and the car will not attain its maximum. There may be back-firing at high speed, but this is not probable, especially if the Choke and Main Jet are according to the factory setting. This back-firing is more often due to large air leaks in the intake or valves or to defect in the gasolene line.

VARIABLE 3 - COMPENSATOR The Compensator is easily removed after unscrewing the Lower Plug. From the explanation of the Zenith Principle given on pages 2-3, it is readily noted that the influence of the Compensator is most marked at low speeds. The Compensator size is best tried out on a hill, as regular as possible and as long as possible, and of such a slope that the motor will labor rather hard to make it on high gear. A long, even, hard pull of this sort taxes the efficiency of the Compensator to the utmost, and will indicate readily the correctness of its size.

Compensator Too Large
Too rich a mixture on a hard pull. It will give the same indication as for rich mixture at high speed on the level.

Compensator Too Small
Too lean a mixture. Liable to miss and give jerky action of the car on a hard pull.

VARIABLE 4 - SLOW SPEED ADJUSTMENT This adjustment is made on the floor, with the motor properly warmed up. It must be remembered that many factors may prevent a good idling, such as:-
Poor gaskets Loose valve stems Pitted valves Leaky valve caps (These first items causing air leaks.) Leaky plugs or priming cocks Spark plug points too close together (See page 8) Fly wheel too light Too much spark advance Late (retarded) spark

The Idling Device differs somewhat with each model and is fully described in the individual Bulletin devoted to that Model. The size of the gasolene orifice is stamped on each idling jet in one hundredths of a millimeter. In the case of Vertical Carburetors, Model L and Model O it may be said in general:-

If Idling Device Is TOO SMALL
It will be impossible to obtain a satisfactory mixture except by turning the Idling (adjusting) Screw all the way in. In this event put in a larger Idling Device.

If Idling Device Is TOO LARGE
It will be impossible to obtain a satisfactory mixture unless the Idling Screw is turned out as far as possible. In this case put in a smaller Idling Device.

General Instructions - Pages 7-8
The following instructions will aid the Zenith user in the care and adjustment not only of his Carburetor but his engine and car as well.
It often happens that symptoms of trouble are very similar whether the source of trouble is in the Carburetor or in some other part of the engine or its auxiliaries. Careful study of the following pages will make clear the remedies for all difficulties ordinarily experienced.

We recommend that before any adjustment of the Carburetor is attempted these instructions be studied in their entirety so that in attempting to make an adjustment you will understand clearly its effect on other points of operation.

The Zenith is very simple in construction, and after a study of the descriptive literature, can be taken apart very easily for cleaning and inspection and no difficulty need be experienced in re-assembly. Every part has its own place and can go in no other. There are no movable adjustments or springs to cause trouble and the re -assembling will not disturb the adjustment in any way.

No matter what care is taken in filling a fuel tank, small particles of dirt & water are bound to get in, and in case this happens the Carburetor should be taken apart and cleaned thoroughly.

Instruction books relating to your engine and car, which are always supplied by the manufacturer, should be carefully studied and particular attention paid to such points as the ignition and valves, their timing and adjustment. Occasional attention to these parts, proper use of lubricating oil and considerate driving will result in satisfaction in the use of the car more than repaying the slight trouble involved.

Learn to anticipate and prevent trouble rather than learn to correct it.

Difficulty in Starting the Engine (1) Page 8
Be certain that the throttle is just open 'a crack' and that the strangler valve is closed tight.
See if there is fuel in the carburetor. If the needle valve is off its seat and can easily be depressed with the finger, the float chamber is empty, and the fuel system should be checked to determine if there is fuel in the tank above the level of the carburetor, or if the fuel valve is closed. If there is sufficient fuel in the tank and the fuel valve is open, take off fuel line and Filter Body of the Carburettor and assure yourself they are not stopped up with dirt.

Take off a spark plug lead and hold it about 1/8" away from spark plug terminal; with ignition switch on, crank the motor and see if a spark jumps across the gap thus formed. If no spark occurs check up the ignition system.

These leaks commonly occur at the joints, and are caused by faulty gaskets, by neglect to place gaskets, or by faulty machining of the manifold flanges preventing their being drawn tight together all around the passage. (The best gasket to use is one of soft, tough and compressible material. Rubber should not be used as gasolene has a deteriorating effect upon it. The gaskets should not be more than 1/16" thick. Thicker gaskets may cause the flanges to become sprung, thus causing another leak.) In the case of tubing manifolds leaks are sometimes found along the seams. In cast manifolds sand holes sometimes occur. To find these leaks use soapy water, or oil, covering the suspected points with it and crank the engine. Disturbance of this film or a hissing sound will indicate a leak.

These commonly occur in older engines are often a source of mysterious trouble until detected and replaced. In case no leaks are found in the manifold, or connections, try squirting gasolene around the valve guides and crank the engine. Too much importance cannot be attached to air leaks as a seemingly insignificant leak will cause serious difficulty in starting the engine and will also seriously impair the running of the engine under closed throttle conditions.

When all of the foregoing points have been attended to and the motor still refuses to start, try the following:-
Prime the engine cylinders through the priming cocks, or, if priming cocks are not provided, take out the spark plugs and prime through the spark plug holes; do not overdo it, a few drops in each cylinder will suffice. Crank the engine and if it starts immediately but stops as soon as the priming charge is burned out remove the Idling Device from the Carburetor and see if it is stopped up with dirt.
If the engine will not start at all remove spark plugs and examine them carefully for cracked porcelains, faulty gaskets, improperly adjusted points etc. The plugs should not have a gap smaller than 0.020" or greater than 0 .025". However, if the batteries or the magneto be weak, starting may sometimes be accomplished by setting the spark plug points a bit closer together, say 0.015" to 0.018". It is desirable in such a case that the batteries or magneto be replaced or repaired.

Other, but uncommon causes of difficult starting are:-
Lack of oil around the pistons, allowing leaks by the piston rings.
In old Spark Plugs the porcelains may be carbonized, or may have very fine fractures allowing a leakage of current. In the case of mica plugs the mica may become oil soaked.

Difficulties with Idling - Page 9
To attain good results in idling an engine, the entire engine assembly must be in good condition and perfect adjustment. Ignition must be strong. Valves must have good tight seats, and valve tappet clearances adjusted as to be certain the tappets do not hold the valves off their seats. This clearance should be about 0.004" in the case of inlet valves and 0.006" in the case of exhaust valves, and should be adjusted when the engine is hot. Otherwise the expansion caused by the heat will change the clearance.
All cylinders should have equal compression, and there should be no air leaks, however small. When these points have been checked and everything found, or made right, the engine should be started and run under slightly open throttle until it is thoroughly warmed up.

If the Mixture is too Rich
It will be indicated by the engine running is a surging and irregular manner. Open the throttle to clear the passage of the rich mixture and then bring it back to the idling position and screw out the Idling Adjusting Screw until regular running is attained. It it is necessary to screw this out more than four or five turns from the seat, put in a smaller Idling Device.
If the Mixture is too Lean
It will be indicated by the engine running in a jerky manner, missing will occur, and in extreme cases the engine will stop altogether. Screw in on the Idling Adjusting Screw until this condition is remedied. In case it is necessary to screw the valve in to within 3/4 to 1 turn from its seat put in a larger Idling Device.
When the proper size of Idling Device has been determined it will be found better to work the stop screw (which prevents the throttle closing tighter than desired) and the Idling Adjusting Screw together in obtaining the "final adjustment". Catch the Stop Screw with a small screw driver, and with the thumb and finger of the other hand grasp the Idling Adjusting Screw, then slowly turn the Stop Screw in or out to bring the engine to its desired idling speed, at the same time turning the Idling Adjusting Screw to make the mixture leaner or richer as may be necessary. Usually it will be found possible to turn out on the Idling Adjusting Screw as the throttle is brought nearer to the closed position. When the desired adjustment has been obtained lock the Adjusting Screws in place.

The FOREGOING refers particularly to Vertical Models. The Idling Adjustment on the Horizontal Models is obtained by turning the knurled screw "up" to enrich the mixture and "down" to obtain a weaker mixture. (See bulletin No 4) In obtaining the final adjustment the idling adjustment and throttle adjustment should be worked together as described above.

NOTE: When making the idling adjustment have it on the "rich" side That is, have the Idling Adjusting Screw adjusted to a point where a slight "turn in" would result in a rich mixture.

Difficulties with pulling away from rest Pages 9-10

Difficulties with Pick-Up Page 10
The mistake of adjusting the Carburetor to admit of quick "starting away" when the engine is chilled through, is quite commonly made. It should be remembered that in adjusting the a Carburetor it is always wise, in fact necessary, to adjust it when the engine is warmed up to its normal working temperature. Therefore, before making a change in the Carburetor adjustment when trouble is experienced in quickly getting away allow the engine to run idle for a few moments to warm it up. The time necessary will, of course, depend on the temperature of the engine and of the air.
In extremely low temperatures it is wise to partially cover the radiator so as to maintain a temperature of from 175 Deg to 190 Deg bin the engine cooling system. This will be found to increase the efficiency of the engine as a whole and will always result in more economical and more flexible action. In case a quick starting away is necessary partial closing of the Strangler Valve will provide the necessary richness of mixture and this may then be released when the engine has attained a temperature where the vaporization of fuel is possible. However, if the normal running temperature has been reached and trouble still persists, it may be due to the following causes:

Idling Mixture Too Rich
This will result in an accumulation of raw fuel in the gas passages and Carburetor. Opening the Throttle will cause this to be drawn up into the cylinders and will momentarily enrich the mixture to a point where it will not explode. Careful speeding up of the motor will usually carry this away, but the obvious remedy is to screw out on the Idling Adjusting Screw, this "leaning" the mixture. This same condition may be caused by insufficient hot air supply, particularly where the manifold diameter is greater than that of the Carburetor, or where the manifold has rough inner walls.
In the former case, close the Temperature Regulator, should it be open, and if not, increase the volume of the hot air supply by fitting a larger stove. The hot air stove should always be in a protected position on the exhaust pipe. If it should be in the path of the cold wind its efficiency will be correspondingly reduced.

Jets Too Small:
Try a larger size Compensating Jet.
Difficulties with Pick-Up Page 10
Defective pick-up may be caused by a Mixture too lean or too rich; by a faulty Spark Plug Adjustment, the points being too far apart; by the Manifold being too large, or having a rough inner surface; by the Choke Tube being too large; and by Insufficient Heating.
If the Carburetor is adjusted to give too lean a mixture the engine will hesitate, spit back or stop altogether when the throttle is quickly opened after running at a low speed. In this case try a larger main jet. (Note: on some engines, due to varying characteristics of timing, manifold size or design, valve size, gas passage size and design etc., the Main Jet will exert a determining influence over this point of operation; in other cases the Compensating Jet will be the one. Therefore try one then the other, selecting, of course, the size Main Jet or Compensator most satisfactory. Try also increasing the heating)

If the Carburetor is adjusted to give too rich a mixture the engine will hesitate, and then run in an irregular manner when the throttle is quickly opened after running at a low speed. In this case put in smaller Main Jet or Compensator. (See note above)

If varying the jet sizes does not accomplish satisfactory results put in a smaller size Choke Tube, selecting jets accordingly.

Assuming that the Carburetor is of the correct size for the engine it may be said that if the manifold is of a larger inside diameter than the Carburetor that considerable difficulty may be experienced in obtaining satisfactory "quick throttle" action. In this case supply all the heat possible as it will tend to counteract the bad effects of the large Manifold.

Lack of Top Speed
This may be due to various causes, and before attempting any Carburetor adjustment, it is advisable to check up the ignition system, being certain that there is sufficient spark advance, check up the brakes to be certain they are dragging. A very small amount of brake surface dragging on the drums will be sufficient to seriously impair the speed and power of the car. Be certain that Tires are pumped up to the point recommended by the manufacturer as the softer the tire the more resistance to motion it will offer. When you are trying for maximum speed you are asking your car to deliver its maximum of efficiency and power, and before it can do this all parts of the car must be in perfect shape and adjustment.
If the mixture is too lean there will be a "spitting back", or a decided hesitation when the Throttle is quickly opened, and in extreme cases, the spitting back will occur at high speed. If there is no spitting back when the Throttle is suddenly opened but if spitting back occurs when the car has reached a high rate of speed then it is a good indication that the supply of fuel to the Carburetor is insufficient to meet the demands of the engine. This may be due to a low level of fuel in the tank, if gravity feed is used; to dirt or water in the Fuel System; to a too low position of the Vacuum Tank if this system of fuel supply is used; or to insufficient air pressure on the fuel if a pressure system is used. In the former case (Vacuum) where spitting back occurs when the Throttle is suddenly opened, a larger Main Jet will undoubtedly cure the trouble.

If The Mixture Is Too Rich
Black smoke will come from the exhaust, in which case a smaller Main Jet should be fitted. If the mixture seems right and still the car will not attain its proper speed it will be advisable to try a larger Choke Tube. This will pass a greater volume of air, and supposing the jet sizes to have been right before, will require one size larger Main Jet.
If the Choke Tube is too large there will be irregular running at high speed, the exhaust "mixing them up" or sounding as though missing occurred, and at lower speeds the action of the engine will be "dead".

Other causes will be restriction of the air entering the Carburetor due to too small tubing being used, or by too sharp bends in the tubing. The cold air entrance to the hot air stove may be restricted, or the hot air outlet to the tubing from the stove may be too small.

With one exception, "Knocking" is rarely, if ever, caused by Carburetor adjustment or action. This exception is when the engine is pulling hard at low speed with a lean mixture, when a knock very similar to a "Spark Knock", and called a "Labor Knock" may occur. Knocks such as the following are caused by various defects in the engine:-
Loose Connecting Rod Bearings - A rather sharp metallic sound
Loose Main Bearings - A dull, heavy knock with accompanying vibration
Loose Pistons - (Piston Slap) A rattling sound, most obvious when running on light load
Ignition Too Far Advanced - A very sharp knock, but without much volume of sound, cured by retarding the ignition
Carbon - Similar to Spark Knock, but cannot be stopped by retarding the ignition, particularly under hard pulling conditions
Labor Knock - Caused by motor running hot or by lean mixture.

Excessive Fuel Consumption Pages 11-12
Because the fuel passes through the Carburetor on its way to the cylinders the Carburetor is most always blamed for any over-consumption of fuel. This is sometimes justified, but more often is not.
Assuming that the Carburetor is properly adjusted so as to give flexibility of running to the motor, allowing it to run in a smooth manner and not showing any tendencies of richness such as "loading up" under a hard pull, foul smelling exhaust, etc., and that it is not leaking, we may safely assume that it is not the source of trouble. The function of the Carburetor is to measure the fuel and the air according to the demands of the engine.

Any engine requires a certain volume of explosive mixture to develop its normal power, this volume being measured by the size of the cylinders, their number, and the number of times they must be filled in order to propel the car a certain distance. Assuming that the engine cylinders have perfect compression; that the ignition is strong and provides the spark at the proper instant; that all bearings are free and lubricated properly; that the brakes are not dragging; that all the tires are pumped up to their proper pressure; that due consideration is paid to the driving of the car etc., we may say that the car will run on a minimum quantity of fuel.

If the compression is weak, if the spark is weak or late, if the brakes are dragging etc., even a small amount, we must use more fuel to make up for the wastage at the weak point. It is essential to go over the entire engine and car before attempting any Carburetor change, unless, of course, the action has been such as to point clearly to Carburetortrouble.

We therefore recommend that suggestions for adjustments relating to the power and flexibility of the car be followed, and it may safely be assumed that the economy - as far as the Carburetor is concerned - will take care of itself.

In making adjustments, other than for idling, keep on the lean side of the mixture, using the smallest jets that will give satisfactory power, speed and acceleration, when the engine is well warmed up. Do not sacrifice power in attempting to gain economy - the adjustment giving maximum power and flexibility is the most economical in the long run.

A good indication of a proper mixture is when the engine will "spit back" when cold, but will give no evidence of leanness after running for several minutes to bring it up to its normal working temperature.

Poor Pulling Up Hills
While it is desirable to have a car fitted with an engine that will pull exceptionally well at low speeds, the tendency of most operators of automobiles is to carry this point too far, resulting in excessive strain and wear on the entire power plant and running gear of the car, as well as actually sacrificing time in the ascent of a hill for the questionable satisfaction of having it go over "on high". This is all very well for demonstrating the power of the car, but for regular every day use it is far better to let the engine run at a speed where it naturally and easily develops its best power.
Under hill climbing conditions the Compensating Jet is exerting its maximum influence on the mixture. Therefore, if the power of the car is low, if missing and a tendency to stall occur, increase the size of the Compensating Jet.

If irregular running of the engine occurs, the exhaust having a "rolling" or "surging" note, and the engine gradually slows down, it is a sign that the mixture is too rich. Therefore put in a smaller Compensating Jet.

A Choke Tube too large will cause irregular running under the above conditions and it will be impossible to secure any combination of jets that will result in smooth powerful running. The velocity of the air past the jets will be too low, causing a "loading" condition to exist even though the jet sizes are comparatively low. This is due to the fact that raw fuel, not sufficiently atomised is drawn into the cylinders causing slow and incomplete combustion. In this case try a smaller Choke Tube, correspondingly reducing the size of the jets.

When the Engine Gets Hot (Page 13

This is indicated by excessive evaporation of water in the radiator, smell of burnt oil etc. The cause my be found in one or more of the following:-
Radiator Problems
Radiator or circulation insufficient, or water jacket too small. In this case radical changes would be required. Lime deposits must be guarded against.

Ignition Too Late.
An engine running under these conditions may get extremely hot. When the defect of knocking is inherent in an engine, one is inclined to diminish the advance, whereupon the risk is incurred of the engine getting hot. It is better to leave the advance alone and close the throttle a little.

Fan Belt Stretched
Fan running too slowly. No way of escape for hot air inside of bonnet, etc. Examine these different features and correct them if necessary.

Influence of Carburetor
In summer, open cold air ports to Carburetor. In Airplanes or Racing Engines running at maximum power and speed over extended lengths of time, a lean mixture will cause overheating.

It is sometimes observed that an engine seems to overheat more with one Carburetor than with another. Assuming that both have been properly adjusted, this may be due to the following causes:-

An engine does not generally get hot when running at high speed, the circulating pump and fan being most efficient under these conditions. Conversely, when the engine slows down in going uphill with full admission, the fan and pump are working more slowly and therefore with less efficiency. It follows, then, that the best Carburetor, ie: the one with a wide range at low speed under load, and allowing the engine to pull more readily when running uphill, will more readily show the imperfections of the cooling appliances.

Given fair "tuning up" of the Carburetor its influences on heating are of an indirect character.

Carburetor Fuel Leakage (Page 13)
When fuel is seen to drip from the Carburetor it is advisable to remove it from the engine and examine closely for leaks at the following points:-
Fibre washers are fitted under each of these, and it may be found that these are defective, or that the part is not properly tightened up. In the case of the Filter Body it may happen that a strand of wire from the Filter Screen has gotten between the gasket and the body and when the Filter Plug was tightened up this wire has cut a groove in the gasket. This can easily be rectified by putting in a new gasket.

FLOODING CARBURETOR It is strongly recommended that the foregoing be carefully attended to before any attempt is made to change the level of the fuel in the Float Chamber. If, after careful inspection it is decided that the leakage is due to faulty action of the Float mechanism, proceed as follows:-

Remove Float and shake it to determine if any fuel has leaked into it. if such is the case, submerge the Float in hot water, the hotter the better, and the action of the heat will vaporize the fuel and it will seek escape at the point of leakage, at the same time showing where the leak is by the bubbles. In soldering up a leak in the Float be very sparing with both heat and solder - too much of the former may open the seams of the Float beyond repair, and too much of the latter will change the buoyancy of the Float or put it out of balance.

See that the Float Chamber is absolutely clean and that no small particles of dirt are secreted. Sometimes small particles of dirt will get under the Needle Valve and embed themselves in the Needle Valve Seat, this being the most common cause of Flooding carburetors. In any event press down on the Needle Valve and give it twisting motion with the fingers, alternately raising and lowering it as when grinding a valve in the cylinder. If this does not stop the leaking try holding the Needle Valve firmly on the seat with the fingers and gently tap it with a light wrench , raising and turning it occasionally. This will usually result in making a new seat for the valve and thus stop the leak.

Examine the valve lever weights. Under vibration the Float will revolve or oscillate about the Needle Valve and the bearing faces of the Lever Weights will become worn flat. When this wear goes so far the fuel will rise to the level of the jets and overflow before the valve is seated. If a decided flat surface is found on the Weights, reverse them, this operation bringing them back to the original point of adjustment. See if the needle valve is tight in the body also.


Adjusting the Float Level Page 14

Adjusting the Float Level
We recommend that, except in cases of emergency, no attempt be made to change the gasolene level of the Carburetor. The instrument should be taken, or sent, to a Zenith Service Station where equipment is provided for such adjustments.
However, it may happen that such a course is impossible at the time and if the following instructions are carefully observed there will be no danger of seriously disturbing the efficiency of the instrument. As soon as possible after changing the level the Carburetor should be sent to a Zenith Station to have the level checked.

Needle Valve Adjustment
The only place any adjustment of the level should be attempted is at the Needle Valve Collar, its relative position on the Needle Valve determining the point of closing of the Valve. Remove the Float Cover from the Carburetor, draw out the wire holding the Lever Weight Axles and remove the Needle Valve entirely from the cover.

Take a knife, or any sharp instrument, and mark around the Needle Valve at the lower side of the Collar in case it is found necessary to bring it back to this position. Scratch a mark on the Needle Valve 1/32" above the collar. provide a block of hard wood through which a hole slightly larger than the largest diameter of the Needle Valve has been drilled. Put the Needle Valve into this with the point down.

Now, with a flame soften the solder holding the Needle Valve Collar on the Valve, first wetting it with soldering acid or paste, and when the Valve is free in the Collar tap it down to the upper mark, or 1/32" .Allow it to cool, thus again setting the solder, and replace it in the Valve Cover. This will lower the level. If a further lowering is found necessary repeat the above but never go further than 1/32" at a time.

Next, replace the Cover on the Carburetor, press the Needle Valve down tight against its seat with the finger and mark a line around it at the upper edge of the boss on the Cover that takes the thread of the Dust Cap. Remove the Cover from the Carburetor and with the fingers bring the mark on the Valve back to the above position and see what position the Lever Weights are set. If they are not horizontal it means that you have gone too far, that the travel of the Valve will be insufficient, and that the Float mechanism will not function properly. This travel should not be less than 5/64".

To raise the level, move the Collar towards the point of the needle valve. Never bend the Lever Weights to change the level, and do not put additional gaskets under the Needle Valve.

Cleaning The Carburettor:
When there is water in the gasolene: a very few drops of water lodged in the bottom of the unions will suffice to produce transient difficulties in carburetion. Assuming the amount of water to be very small, the trouble may be remedied by taking out the two hexagon nuts or plugs under the jets and the filter plug and emptying the Carburetor.

When Jets are Choked With Impurities:

This is of frequent occurrence when there is no filter between the gasolene tank and the Carburetor; and sometimes even with a filter. A tiny speck of dirt etc. ,may partially clog the aperture of a jet, and though the engine continued to work, it does so imperfectly, giving indications of defective carburetion; popping, especially when picking up speeds etc.

The Jets can often be quickly cleaned out by closing the air strangler for a few seconds when running the motor fast. Take out the jets and remove any impurities found, taking care not to widen the apertures by using a cutting tool; clean out the Float Chamber, the pipes etc.



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