See Figure 1
In addition to the intake valve and the exhaust valve, a jet valve has been added to draw air (super lean mixture or air) into the combustion chamber. The jet valve assembly consists of a valve, body and spring which are all screwed into the jet assembly and pressed into the cylinder head with the jet opening directed towards the spark plug. The jet valve draws its air from a passage in the carburetor, intake manifold and cylinder head. The air flows through the intake openings near the primary throttle valve of the carburetor and into the intake manifold and cylinder head, it then flows through the jet valve opening and into the combustion chamber. The jet valve is operated by the same cam as the intake valve and is attached by a common rocker arm so the jet valve and intake valve open and close at the same time.
On the intake stroke, the air/fuel mixture flows through the intake valve port and into the combustion chamber. At the same time, jet air is forced into the combustion chamber as the piston moves down. The jet air running out of the jet opening scavenges the residual gasses around the spark plug and creates a good secondary ignition condition. This also produces a strong air flow in the combustion chamber which continues throughout the compression stroke and improves the flame spread after ignition thus assuring a high combustion efficiency. The jet air flow dwindles with the increase of the throttle valve opening but the intensified inflow of normal intake air mixture can properly promote combustion.
An incorrect jet valve clearance would affect the emission levels and could also cause engine malfunction. Adjust the jet valve clearance before adjusting the intake valve clearance. Furthermore, the cylinder head bolts should be tightened before making this adjustment. The jet valve clearance should be adjusted with the adjusting screw on the intake valve side fully loosened.