The component on the bottom left is a solenoid valve which quite rigorously switched off the idling system exactly then when ignition current was no longer available. It has proved to be a good solution, at last, a successful measure to prevent overrunning by self-ignition. Even without any electric ignition, the engine repeatedly sucks an ignitable mixture into the combustion chamber, which is then ignited by any possible glowing particle. Quite often it can only be stopped by stalling the engine, which of course, is not the ideal solution.
Because the ignition is switched on, the idling channel is now open. We are assuming a cold-starting with the choke-flap shown right at the top, completely closed,. The air supply from the air filter is thus, pretty much shut out. So that additional vacuum at the top is effective, the throttle-flap at the bottom is slightly open. From the here developing fuel portion, just enough is deposited on the inlet- and cylinder walls, that a slightly richer mixture is available for combustion.
As soon as the engine starts, an increased vacuum develops in the pipe on the right, which slightly opens the choke through a spring-loaded membrane.
Here the engine carries on idling, indeed, only for the purpose of distinguishing the functions of the individual components better. A bi-metal spring now provides for the further opening of the choke. Through the two metals, which have differing thermal expansion values, the tension in the spring is relaxed when warming up. It is more favourable if the actual temperature of the engine is recognised through the circulation of its coolant, than if it is electrically warmed up and time-controlled.
At the same time the throttle-flap, by the way, goes back to its warm idling position by means of a notched disc. The precise position is reached, when the choke-flap is in a vertical position. Now the throttle-flap is closed just enough that it doesn't stick when opening. The air, somewhat enriched with fuel for the idling now comes in as pre-foamed fuel through the larger channel and past the thicker adjustment screw, through the smaller channel and past the thinner mixture regulating screw.
This condition is called the 'transition-phase'. This is where, e.g., when driving off, a bit of gas is given. Now, there are three drillings, mostly situated one above the other in the area where a vacuum is created by the engine. The pre-foamed fuel is drawn from the idling system, the amount is larger than when idling because it's not limited by the mixture regulating screw.
We are now in the middle of the partial-load area. The vacuum in the venturi in the middle of the mixing chamber, is sufficient to draw the fuel/air mixture from the three nozzles in the float chamber. The amount is limited by the main jet directly below the nozzle. At the same time, with a larger withdrawal, so-called braking air comes in from above preventing an over enrichment.
Now more gas is given. The arrow in the right of the picture indicates that pressure is applied to the accelerator pump by a linkage, which is not shown here. The piston moves downwards and provides for additional enrichment, this is done by an injection pipe which leads to the mixing chamber found above. Indeed, this only lasts, depending on the movement of the gas pedal, for a certain length of time.
In the partial-load area, fundamentally a rather lean mixture was produced in favour of a lower fuel consumption. Now at full-load, the specified performance should be achieved. The additional small pipe, shown here for the first time, is of course always there, it was only left out before to increase the clarity. The vacuum in the area of the mixing chamber and somewhat above it, is so strong, that additional fuel can be drawn from the float-chamber by the small pipe. 02/15