These idle jets would supply fuel until larger throttle openings allowed sufficient airflow to cause the main jets to come into play.
These added extra fuel that the main jet was not able to supply during high load conditions.
As ever-larger, more powerful engines came into being, it became clear that single throat (or barrel) carburetors could not provide a proper mixture throughout the speed range.
The Zenith Stromberg is considered by many to be an emission carburetor design.
This is somewhat of an unfair label, as my 1964 Morgan had a pair of Zenith Strombergs before emission laws even existed and the engine did not lack for power.
Another advantage of the SU carburetor design, i.e.
varying the carburetor opening due to engine demand, is that full throttle can be applied at low speeds without the causing the engine to bog, as is usually the case with conventional carburetors.
The Weber carburetor is popular with a lot of high performance cars whether as stock or added on by the owner.
The Weber does have the advantage of being very tunable and when correct can give very good results, especially with very high performance engines that dont have a very smooth vacuum.
Therefore the Weber is usually used on high performance engines, and since there is no standard setup for non Weber equipped engines, the results is a lot of trial and error to see what works. This carburetor operates on what is known as the Constant Depression principle.
Depression is Brit-speak for vacuum and thus SUs and related carburetors can also be called constant vacuum carburetors.
(The Zenith Stromberg carburetor is also a constant depression carb.) These carburetors are designed such that the throat or opening varies with engine load, resulting in a constant depression or vacuum being maintained at the jet opening.