I know that vacuum does (duh), but more specifically, how is it adjusted? In other words, how does the carb 'know' to open the secondaries once it reaches a certain vacuum, and what part of the carb is responsible for doing this (I need to know what part to change in order to get the secondaries to open sooner/later)? I apologize, but I haven't gotten my book on Holley carbs yet.

 

Using a lighter or heavier secondary spring will make the secondaries open sooner or later.

 

Holleys have a ported vacuum source that goes to the vacuum bell on the secondaries, as opposed to a manifold vacuum source. That means that the more air flows past it, the more "pull" it has. Ported vacuum is always 0 at idle, but increases as rpm does. This pull is resisted by a spring which can be changed out, which lets you tailor how soon the secondaries open reletive to the needs of the motor.

 

I'll agree except for the part about it being in the secondaries. The port is in the primary venturi area (see http://www.holley.com/HiOctn/TechSer...fo/TI-223.html ).

 

What 57 said. It's air flow through the "primaries" used to open the secondaries that is regulated by a spring.

 

I should have said "secondaries only", since the port goes to both the primary and secondary venturis.

Read the Holley info, and the secondary port "adds to that in the primary verturi to give a stronger signal to the diaphragm".