The function of an accelerator pump in the carburettor of a piston engine is to enable proper functioning of a piston engine during rapid throttle advance. In fact, a rapid throttle advance will translate into an abrupt huge air intake into the carburettor. However, the fuel metering unit of the carburettor is not as responsive, so there will be less fuel than air, which will result in a temporary weak mixture. The accelerator pump is mechanically linked to the throttle, so as soon as it is moved, the pump will force fuel into the venturi to achieve the right mixture for the given throttle setting.