How do the secondaries on a Holley 4V work?

Vacuum secondaries open gradually as demanded by air flow.Snapping the throttle open will not actuate them.The concept of vacuum operation is to only provide enough airflow to meet engine demands.On Holley carbs when the airflow through the primaries reach a given point vacuum is created in a pickup tube in the venturi which is applied to the top portion of the secondary diaphragm pulling the secondary throttle plates open. As the engine speed increases so does the vacuum in the venturi. This puts more pull on the diaphragm further opening the secondaries.Now as more air flows through the secondaries vacuum is created in a pickup tube in the secondary venturi pulling the diaphragm to the fully opened position
At low engine speeds the secondary pickup tube functions as a bleed to prevent the operation of the secondaries when not needed. The bleed and the check ball in the diaphragm housing allow a smooth and steady opening of the secondary throttle plates.

When you momentarily open the throttle (high rpm) in neutral you should see the secondaries open. The two biggest problems with vacuum secondaries is matching the diaphram spring with the available vacuum, and secondaries just stuck closed. Different size springs are available from Holley to fine tune the point at which the secondaries open. I have come to appreciate mechanical secondaries and favor the 650 double pumper. The only drawback is that it only comes with a manual choke. I find the mechanical secondaries to be 100% reliable, but realize if we all liked vanilla they would not make strawberry.

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