The Engine is said to be idle when an external loads is applied on the engine and at this condition the throttle valve is almost closed.

Under the idling condition the power developed by the engine is just sufficient to overcome the various friction losses of the engine. Low load running is usually taken in the range of zero to 20% of the rated power engine.

At the time of starting and idling the engine, the working temperature are low. Therefore the carburetor is not able to vaporize the fuel and the mixture reaching the cylinder is lean. This may lead to non-initiation of combustion in the cylinder.

To ensure minimum fuel vapour in the cylinder rich mixture are necessary to initiate the combustion.

Further, under the conditions of idling and low load running the throttle valve is almost closed due to which the pressure in the intake manifolds is much lower than atmospheric pressure while the pressure inside the cylinder is approximately atmospheric pressure at the end of exhaust stroke.

When the inlet valve opens during the suction stroke, there shall be back flow of residual gases into the intake manifolds.

When the piston moves outward, the residual gases are drawn along with the fresh charge. Therefore the actual mixture inside the cylinder would contain large percentage of residual gases in the fresh charge i.e. the cylinder mixture is to dilute and it is at low temperatures. This diluted mixture is not able to initiate proper combustion.

In order to offset the dilution of fresh charge due to residuals gases and low temperatures, it is necessary to supply rich mixture during starling, idling and low load running of engines.

Usual Air-fuel ratio requirement about to 11 to 12:1.