See Figure 1
Although a mechanical component of the valvetrain, the jet valve (See Engine And Engine Overhaul ) is present to improve combustion and thereby reduce emissions. The jet valve admits a super-lean mixture (almost all air with no fuel) into the cylinder at the same time the intake valve admits the normal air/fuel charge.
When the throttle position is almost closed, such as at idle or light throttle, a large pressure difference is created within the cylinder, drawing in large amounts of jet air. This rapid flow swirls in the cylinder, scavenging the remaining exhaust fumes near the spark plug and creating a good combustion environment. The swirl within the cylinder continues during the piston's compression stroke and aids in more complete combustion.
As the throttle opening is increased, the volume of jet air is proportionally reduced, but the greater volume of air entering through the intake valve is sufficient to promote good combustion.
As with most other emission control systems, there are times when the leaning effect of the jet valve is undesirable. A cold start, with the choke engaged, requires a rich mixture to allow the engine to run smoothly. The jet valve still opens with each intake stroke (it's driven by the camshaft) but the air passage is blocked by the jet air volume control valve. The valve is controlled by coolant temperature; when the engine warms to a pre-determined point, the temperature sensor activates the jet air volume control valve and the air flows into the cylinder on each intake stroke. The temperature sensor controlling this system is the same one used to control the EGR valve.