Mostly true, Dave.

In a two-needle carburettor:

1. There is only one fuel circuit and it is used to supply fuel to the engine, over its entire operation range.

2. ...But, this circuit has two controls, placed in series to one another. They are controlled obstructions to the flow of fuel.

3. The first is 'the high-speed needle', which for the great majority of R/C carburettors, is fixed, once set by the user (the effect of higher fuel pressure at high throttle is minuscule).

4. The second is 'the low-speed needle', or 'fuel metering valve', which changes its degree of flow obstruction, as the throttle position is changed, making it easier for the fuel to flow as the throttle is opened further.


The second control thus affects the amount of fuel drawn by the engine, as long as it is a greater obstacle to its flow.
The first control takes over, only at very high throttle positions, as the second control ceases to impose a limitation to flow; and thus controls ONLY the maximum flow of fuel the engine can draw.

Providing the high-speed needle is set for a peaked top-end, opening it further, when the engine is at a part throttle setting (i.e. fuel supply is limited by 'the low-speed needle-nozzle interaction'), will hardly have any effect on the mixture strength. Closing it will not have much effect either, until it; instead of the low-speed needle-nozzle interaction, becomes the greater obstruction to fuel flow...
Closing the throttle further will again 'cancel' the effect of the high-speed needle.


In engines with two-needle carburettors that have full-range fuel metering, those that employ a longer nozzle with the fuel orifice at the side of the nozzle, in the middle of the venturi; such as the ST, or the MVVS carburettors; it is possible for the low-speed needle's position, if it is closed too far, to make it impossible to adjust the high-speed needle for full throttle operation.
Opening the high-speed needle more will have no effect in such an instance, like at a part-throttle in other carburettors.