Spark Timing Control System

Fig. 25 Vacuum controlled spark timing control system. (Typical):

Fig. 26 Electrically controlled spark timing control system. (Typical):

Refer to Figs. 25 and 26 for typical spark timing control systems. Complete system schematics are located in ``Vacuum Hose Routings.''




The spark timing control system is designed to control distributor vacuum advance in order to reduce HC and NOx emissions. The thermal vacuum valve opens and closes in response to coolant temperature, causing the spark timing control vacuum line to be exposed or closed to the atmosphere. When the valve opens, air enters the vacuum signal line, preventing distributor vacuum advance.
The vacuum control valve is installed on the distributor vacuum line. When carburetor venturi vacuum exceeds a pre-determined value, air is bled to distributor vacuum line and the spark timing is retarded slightly.