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Today while doing some minor engine stuff, I decided to check the operation of the carb. While the car was running, I grabbed the throttle linkage and gave it a nice hard pull to simulate flooring the pedal.

Nothing happened at all with the secondary plates. The secondaries are vacuum operated. I was thinking that maybe, because I had the air cleaner off and so there wasn't any vacuum, that might be the reason. I think that the vacuum for the secondaries pump comes from a small vacuum hole at the top of the venturis, but I'm not sure.

So how do the secondaries work? Is my vacuum thinking right or do I have non-functioning secondaries right now? With the air cleaner on, I can't really see the secondary throttle linkage to be able to tell if it's working.

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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.
 
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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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