Exhaust Gas Recirculation: Description and Operation

Fig. 2 EGR system:

The EGR system, Fig. 2, lowers combustion temperatures within the combustion chambers, reducing nitrogen oxide emissions.
Exhaust gas is drawn from the cylinder head exhaust port into the intake manifold riser portion, through the cylinder head passages, intake manifold and EGR valve. The EGR valve vacuum diaphragm is connected to the carburetor flange signal port through a thermal vacuum valve controlling EGR cold override.
Opening the carburetor throttle valve applies a vacuum to the diaphragm, in turn opening the EGR valve to meter exhaust gas flow into the intake manifold.