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Holley 1901 Spark Control Valve

Holley 1901 Spark Control Valve

All manifold vacuum to the distributor passes through the spark control valve. Under normal road load conditions, the spark valve is held open against the pressure of a pre-set factory calibrated spring by a combination of atmospheric pressure outside of the diaphragm and manifold vacuum from within. When accelerating, manifold vacuum momentarily drops below a predetermined point and the calibrated spring closes the spark valve, shutting off the largest of two passages through the spark valve. The passage left open is a caliĀ­brated by-pass which permits a small amount of manifold vacuum to reach the distributor vacuum line. Closing the valve prevents excessive spark advance, while the manifold vacuum passing through the spark valve by-pass prevents full spark retard. As engine speed approaches the throttle setting, manifold vacuum increases sufficiently to open the spark valve and allow a higher vacuum to operate the distributor. This carburetor also employs a distributor passage ball in the manifold vacuum passage to prevent weakening of the vacuum to the distributor whenever venturi vacuum beĀ­comes stronger than manifold vacuum. By use of the spark valve, more efficient spark advance upon acceleration is obtained without the excessive “ping” of the normal road load part throttle spark advance or the sluggishness of wide-open throttle spark retard.
The spark valve functions in a similar manner to provide an intermediate spark retard whenever the load on the engine is increased to a degree where the normal road load spark advance would be too great and the wide-open throttle spark retard would reduce the efficiency of the engine.

Updated on 09/20/2021

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