EGR Valve
On 1974 non-California models, the EGR system does not receive sufficient vacuum to operate at idle. System operates during deceleration and part throttle. As a result, no exhaust gas recirculation occurs during idle and low intake manifold vacuum.
On 1974 California models, vacuum to shut-off valve varies with throttle opening and valve begins to open when 2.8 in. Hg of ported vacuum is applied. Regulating valve is operated by manifold vacuum and is fully open between 3.5-9.8 in. Hg of vacuum. The operation of these 2 valves provides no exhaust gas recirculation during idle, a little recirculation at low engine loads, and maximum exhaust gas recirculation at high engine loads.
On 1975-79 models, vacuum diaphragm chamber of EGR valve is connected to vacuum port in carburetor flange through backpressure transducer (if equipped) and thermal vacuum valve (if equipped). As throttle valve is open, vacuum is applied to EGR valve. When enough vacuum overcomes EGR valve spring force, it opens EGR valve to allow recirculation.