Vacume Spring??

They do open as you accelerate from idle but the rpm at which they start to open is quite a bit above idle rpm, guess my statement was confusing.

If you pick up a set of the Holley secondary springs they have a little chart that gives you an approximate starting rpm and rpm for full open of the secondary by spring color and engine displacement. This like is to the instruction sheet that comes with the springs and has the table.
http://www.holley.com/data/Products/Technical/199R8219-2.pdf

With a 350 engine the starting rpm is around 1600 with a the short yellow spring and over 2000 with the plain. It would be higher with a 318 because it doesn't draw in as much air as the bigger engine.