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 millimetres according to the size of their smallest inside diameter.
The Jets and Compensators are numbered in hundredths of a millimetre. A 'one hundred' jet has one millimetre hole and is smaller than a one hundred five jet. The sizes vary regularly by steps of 5 hundredths of a millimetre.
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.
VARIABLE 1 - CHOKE TUBE
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 rom 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 millimetre. 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.