The needle plays a fundamental role in the calibration of a carburettor, in fact it has the task of managing the quantity of petrol that is delivered from the maximum circuit at the different openings of the accelerator. The carburettor valve controlled by the accelerator (commonly known as the gas or throttle valve) determines, on the one hand, the amount of air that the engine can suck in, and on the other, the pressure drop required to draw petrol from the carburetor circuits.

Finding the needle that allows to obtain the optimal air-fuel ratio at all accelerator openings is decidedly complex, in fact each engine has specific characteristics which uniquely determine the intake air flow at the various accelerator openings and consequently also the depressions that will be created in the carburetor will be distinct from engine to engine.