Autolite SCV Spark Valve

What the spark control valve does?

The spark control valve is often mistaken as a power valve, but it doesn’t have anything to do with powering up the carburetor and everything to do with helping out the distributor. For engines that use the spark control valve, the distributor advance is controlled by the engine vacuum and the carburetor venturi vacuum.
In normal operation the manifold vacuum works against the spring in the spark control valve and holds the spark valve open. While the valve is open, the manifold vacuum aids the venturi vacuum for the distributor advance.

When accelerating manifold vacuum starts to drop and the calibrated spring in the spark control valve begins to close and shuts off the vacuum to the distributor, which retards the timing. Venturi vacuum acts as sort of a buffer zone, which keeps the distributor from doing a complete retard. As the manifold vacuum again increases, the spark control valve starts to open again thus advancing the distributor.

Some carburetor types may have a version of carburetor with the spark control valve and a version without. A couple of carburetors that come to mind is the Holley 1904, 1 barrel carburetor and the Autolite 1100 carburetor. You cannot mix the two. Carburetors with a spark valve were the earlier edition and carburetors without were the later edition. The later edition carburetor has the functionality of the spark valve built in.

If your vehicle was designed to use the spark control valve then it needs to be there.

I have been told it doesn’t matter but that just isn’t true. You can’t expect the distributor advance to work correctly with a carburetor designed to use a spark valve that is plugged off.

Updated on 01/11/2021

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