Compensating Jet:

Fig 1. Shows the arrangement of a compensating jet device. Its function is to make a mixture leaner.

Fig. 1. Compensating Jet

It consist of an additional compensating jet connected to compensation well which is open to atmosphere.

The fuel is supplied from float chamber through a restricted orifice.

When the air flow increases the fuel level in compensating well decreases. It results into decrease of fuel supply through compensating jet while the main jet increases the fuel supply.

Net results of both the jets is to maintain the constant A/F ratio.

The effect of compensating device on air-fuel ratio is shown in Fig. 2.

Fig.2. Effect of compensating jet on A/F ratio.

Idling jet

Fig.3.Idling Jet

Main metering and idling System: It has been stated that idling of the engine refers to no load running of the engine and the engine requires a rich mixture.

At no load the throttle valve is almost closed and due to this the airflow through the venturi is greatly reduced. It means that the pressure drop at the venturi is very low and the main jet is not able to supply any fuel.

In order to supply rich mixture an idling circuit is introduced in the carburettor as shown in Fig. 3.

It could be seen that the idle port is just located below the throttle valve.

The low pressure existing in the intake manifolds past the throttle valve allows the fuel to be supplied from the float chamber through the idle jet thereby enriching the mixture of fuel and air.

The air-fuel mixture discharged into the air stream past the throttle valve is controlled by the idling adjustment screw.

One or more air bleeds are provided in the upper passages as shown in Fig. 3. These air bleeds prevent the fuel to be supplied from the float chamber when the engine is shut-off.

When the throttle valve opens, the pressure differential between the idling port and the float chamber reduces. This reduction in pressure is not sufficient to lift the fuel additional height upto the idle jet.

Therefore at part load running the idle jet becomes ineffective.