holley carb
You will notice that the secondary throttles are locked out until the primaries are nearly wide open.Vacuum to operate the secondary throttles is picked off a small port in the primary venturis and the passage from those ports to the vacuum diaphragm is internal to the carb. No external vacuum source is required and no provision for it is made.
Once the primary throttle goes nearly wide open the secondaries are unlocked and go wide open as well; there is no way to throttle the secondaries. It's all or nothing. There are models of Holley carbs that have mechanical secondaries that do have a measure of secondary throttle-ability.
Look down into the narrowest part of the primary venturis and you will see the small hole. It's about 0.02" in diameter. You will want to make sure that the passage from that hole to the vacuum port on the diaphragm assembly is clear.
Last edited by Kattumaram; Aug 13, 2010 at 10:38 PM.

