Ignition System: Description and Operation

OPERATION
Manifold vacuum and carburetor ported vacuum are both used for the ignition spark advance mechanism. On some engines, a coolant temperature override (CTO) valve determines the correct vacuum source, according to coolant temperature.

On other engines, a non-linear vacuum regulator (NLVR) valve combines manifold vacuum at idle speed and carburetor ported vacuum at a ratio proportional to the amount of throttle opening.