[robert]

There are a number of ways to tune the moment when the vacuum secondaries open on a Holley four barrel carb. The vacuum secondaries are controlled by a diaphram and a color coded spring. Holley makes a number of different springs with different tension on the springs. You can change the springs and change the opening moment. The color coded springs run from light tension to heavy:

White - Lightest
Yellow (Short Spring)
Yellow
Purple
Plain (Steel grey)
Brown
Black - Heaviest

If there is a bog or hesitation when the secondaries open, the spring tension is too light, go to the next heavier spring. Holley offers a kit (PN-20-13) which contains one each of the above color springs. When you change springs you'll note that the stock cover over the spring and diaphram is not all that easy to get to. Holley makes a special cover (PN-20-59) that makes spring changes quick. Finally, Holley also makes a completely adjustable thumbscrew operated diaphram cover (PN-20-99). This cover limits the travel of the diaphram and therefore limits how far the secondary throttle plate can open. It makes secondary throttle opening adjustable. It's very nice to have if you are a bracket racer and are using a carb with vacuum secondaries. You can adjust the throttle for changes in weather and track conditions or for changes in your "dial in".