What is a "double-pumper" carb?

[Anthony]

Originally Posted by Sledge

For example, it looks to me like carbs with vacuum secondaries (like my Street Avenger 770 cfm) have power valves, while (at least some) carbs with mechanical secondaries seem to have accelerator pumps. It looks to me like the power valves and the accelerator pumps essentially perform the same functions, but again, I can't really tell based on my web searches.

So, what is a "double-pumper"? Does the term apply to carbs with mechanical secondaries only?

Originally, 4 bbl carbs were mainly vacuum secondary, that is when you opened the primary throttle to a set point, then the secondaries would open to allow more air flow with little restriction. At low throttle, low power operation, having all 4 bbls open would have less air flow through each venturi, so running off of 2 bbls at low throttle instead allows better fuel metering.

As said previously, a vacuum secondary carb uses the manifold vacuum, with the holley, via a vacuum dashpot mounted on the side of the carb, that pulls the secodary throttle plates open at a certain rate, allowing the fuel to start flowing into the engine along with air. By changing some settings, such as springs, you can adjust how fast the secondary circuit opens to maximize performance. Basically you want the fastest opening without bogging the engine.

When you let up on the gas to shift, both the primary and secondary circuits close until you step on the gas again. The promary opens immediately, but since the secondary is vacuum, it is slightly delayed. Every time you let up on the gas, and then step on it again, there is always that slight delay for the secondary to open. With an automatic tranny, when you accelerate hard, (or with power shifting a manual) you never lift off the throttle between shifts, so once the secondary is open, it stays open.

With a double pumper, the secondaries have an accelator pump as well as the primary, so when you first step on the throttle, both the primary and secondary open immediately, so there is no slight delay as experienced with a vacuum secondary.

As I see it, the only real advantage of a double pumper is with a manual tranny car that you lift on the throttle between shifts, with the double pumper, you may have an advantage over a vacuum secondary carb.

The power valve involves metering fuel under a slight load condition, and really is not related to whether a carb is a vacuum secondary or a double pumper, as they both have power valves unless you remove them.