Carbureted Engines

The vacuum signal is ported above the idle position of the throttle valve to eliminate EGR at idle. During cold engine operation the thermovalve is open, vacuum signals to the EGR valve are vented to atmosphere and the EGR valve is closed for cut off.
When the engine coolant temperature exceeds the set temperature of the thermovalve, ported vacuum is introduced through EGR control valve A into the EGR valve. This opens the EGR valve allowing exhaust gas into the intake manifold. EGR control valve B is normally closed. When manifold vacuum reaches the set level the valve opens allowing venturi vacuum to enter control valve A and the EGR valve. This allows the amount of vacuum applied to the EGR valve to be continuously varied in accordance with engine load.