Single Jet Action
To illustrate this principle briefly let us consider the elementary type of carburettor, 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 gasolene 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 carburetor, while the amount of gasolene 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 carburetors, 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 gasolene 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.