Emission Control Systems: Description and Operation

Fig 22 Typical Jet Air System:

In addition to the intake valve, a jet valve is used for drawing air into the combustion chamber, Fig. 22. The jet valve assembly consists of a jet valve, jet body and spring, which is threaded into the jet piece.
The jet piece is pressed into the cylinder head with the opening in the jet piece facing the spark plug. A jet air passage is provided in the carburetor, intake manifold and cylinder head. The air flow enters the two jet air intake openings on the primary side of the carburetor, one is located above the closed throttle while the other is closed off by the throttle valve at idle speed. When engine is idling or under light throttle applications, the pressure difference between the jet air opening in the carburetor and the combustion chamber during the intake stroke is great, thus causing a high rate of jet airflow to the combustion chamber. As the throttle opening is increased, the pressure differential between the combustion chamber and jet air passage is decreased, causing a reduced jet airflow to the combustion chamber. However, the intensified flow of the normal intake air/fuel mixture satisfactorily provides for efficient combustion.