Idling System

It has already been shown that at idling and low lead, the engine requires a rich mixture (about A/F 12:1). However, the main metering system not only fails to enrich the mixture at low air flows but also supplies no fuel at all at idling. For this reason, a separate idling jet must be added to the basic carburettor. An example of idling jet is shown in Fig. 10.30. It consists of a small fuel line from the float chamber to a little on the engine side of the throttle. This line contains a fixed fuel orifice. When the throttle is practically closed, the full manifold suction operates on the outlet to this jet. In addition, the very high velocity past be throttle plate increases the suction locally. Fuel can, therefore, be lifted by the additional height up to the discharge point, but this occurs only at very low rates of air flow. As the throttle is opened, the main jet gradually takes over while the idle jet becomes ineffective. The desired air-fuel ratio for the idling jet is regulated manually to idle adjust, which is a needle valve controlling the air bleed.