Exhaust Gas Recirculation
A vacuum line, connected above the throttle valve of the carburetor, connects to a diaphragm type control valve. When engine is idling, no vacuum is present and the control valve remains closed. Above 2000 RPM, vacuum is sufficient to open the control valve and exhaust gases are then metered through the valve into the intake manifold. Vacuum reaches the highest level at about 3500 RPM, and at this point the control valve will be open to its maximum extent. A maximum quantity of exhaust gases are allowed to flow into the intake manifold during this time. At speeds greater than 3500 RPM, and at full throttle, the vacuum valve closes and prevents the entry of exhaust gases into the intake manifold.