The functions of the carburetor are:

1) To control the amount of power produced by the engine by modulating the amount of air-fuel mix that it can aspire. This is achieved through the position of the throttle’s barrel.

2) To provide the engine with a proper mixture of fuel and air for any position of the throttle.

Number two is easier said than done, because the engine needs a different mix proportion for each regime of rpm and load.
For idle, the glow engine needs 3 parts of air per 1 part fuel
For middle rpm’s, the glow engine needs 4 to 5 parts of air per 1 part fuel
For max rpm’s, the glow engine needs 6 parts of air per 1 part fuel

A perfectly modulating carburetor will be able to provide the perfect mix for each rpm regime, besides compensating for changes in air temperature, pressure and humidity (that is what the computerized fuel injection system of your car does). Unfortunately, such carburetor will also be complicated, expensive and heavy.

For our simple RC engines, which are fed by more rudimentary carburetors, the proper mix for each rpm is a fine balance that is difficult, but not impossible to achieve, if all the engine’s parts and fuel are working properly.

A well-tuned and good carburetor will provide a proper combustion through a balance between fuel and air that is good for idle, transition and max rpm’s.

The vacuum created by the flow of air through the Ventury next to the throttle is the force pulling the fuel out of each spray-needle into the mix.

Think of the low-end needle like part of a little carburetor that mixes the minimum airflow of idle with the proper fuel amount.
That ideal fuel amount is what we adjust by turning the low-end needle.

Think of the high-end needle like part of a bigger carburetor that mix the max airflow of full rpm’s with the proper fuel amount.
That ideal fuel amount is what we adjust by turning the high-end needle (and the low-end needle, which also provides the small amount of fuel that is proper for idle).

The middle rpm’s regime is satisfied with the combined mix from both carburetors, the little one for idle and the big one for max rpm’s.