Riggin' The Secondaries
Q

I read your article about vacuum-secondaries not completely opening at WOT. Could I put a small bolt and nut in the slot on the secondary shaft to open them all the way, sort of mechanically, or would this cause some problem? The carb is a Holley 4150 850-cfm on a GM 502 Deluxe Crate engine. Thanks.
Paul Bourke
Lockport, IL

A
We used to do this back in high school to make the 780-cfm Holleys a mechanical-secondary. Several aftermarket companies would sell add-on linkage to convert vacuum carbs to mechanical secondaries. The linkage you're referring to has a specific purpose. It is to close the secondaries if the spring in the vacuum secondary diaphragm isn't strong enough to close the secondaries. This is a very important function and shouldn't be taken lightly when you're modifying the carb. There's nothing worse that a runaway car with a hung throttle!

We searched around for quite some time but couldn't find any of the companies we remembered selling these components. The main problem with converting a vacuum-secondary carb over to a mechanical-secondary is that you don't have a secondary accelerator pump. In the dyno cell, when you can open the secondaries slowly at full engine speed, this isn't a problem. However, from an idle, if you try to open all four barrels without a pump shot in the secondaries, you will have a tremendous bog. You could try to cover this up with a larger accelerator pump diaphragm (50 cc) and a larger accelerator pump nozzle, but this causes its own set of problems. Trying to drive the car on the street with that much shot going in every time you step on the gas would be way too rich.

Why don't you get a secondary spring tuning kit from Holley and tune in your secondary opening? Holley's Secondary Diaphragm Spring Kit (PN 20-13) gives you a selection of seven springs to tailor the secondary opening right to your engine. Check out the spring that was delivered in your 850-cfm Holley and look at the chart supplied with the springs. Move to one spring lighter until you pick up a slight bog from a hard launch. Then back it off one. To make the job very quick and easy, the Quickchange Vacuum Secondary Housing Cover (PN 20-59) makes it a snap to change out the springs. By tuning in your secondary opening you can get very close to double-pumper performance. Work this out first before you rig your secondaries.

Source: holley.com