The spark control valve in the passage from the throttle bore provides an efficient degree of spark advance during periods of acceleration by momentarily providing a partially retarded spark. This partially retarded spark minimizes detonation, or pinging and increases engine responsiveness while accelerating. In effect, the spark valve provides an intermediate spark advance curve between a fully retarded spark and the normal spark advance which would be excessive when the engine is suddenly accelerated. The spark control valve accomplishes this by controlling the manifold vacuum admitted to the distributor vacuum line.
