What is a "double-pumper" carb?

[Sledge]

This week's dumb question:

I see the words "double-pumper" used frequently when discussing carbs. I've been doing all kinds of searches on the web, various carb manufacturer's sites, etc., but have not seen a definition.

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?

Is my Street Avenger a "double-pumper"?

Thanks in advance

[Woodz428]

2 terms that are often confused in some circles are Dual feed and double pumper. A double pumper is a mechanical secondary carb with 2 accelerator pumps, one for the primaries and one for the secondaries. That way when you kick open the secondaries there is no bog or pop back. A dual feed just means that there is and external fuel line to both bowls insead of a small tube leading from the front bowl to the back one. I don't know for sure which carb you have but I believe all Street Avengers are vacuum secondary and would not be a double pumper.

[Velox]

Hi Sledge, the "double pumper" refers to the accelerator pumps, with a single pump located on the primary side of the Holley in carbs with vacuum secondaries. A second accelerator pump can be found on the secondary (rear) side of the Holley when mechanical linkage is used. This is a picture of my 850 Holley double pumper, note the lack of vacuum linkage for the secondary side and the vertical spring and linkage just in front of the rear fuel connection which is the rear accelerator pump linkage - pumps in front and back = double pumper

[Sledge]

Thanks WDZ!

[Ron61]

Sledge,

The others have really covered this well, but if you have a double pumper you can tell easily with the motor off by just opening the throttle wide. You will see the gas squirt into the secondaries just like the primaries. Mine has a way to adjust where I want the secondaries to come in by changing a small cam link.

Ron

[Excaliber]

It's like flushing the toilet, twice! When I was getting 6 mpg it sure felt like thats what I was doing.

Now I have a smaller cam and smaller vacuum secondary carbs, 10 mpg. And it STILL runs the 1/4 in the 11's. Go figure...

[Sledge]

Thank you Don, Ron, and Excaliber, good stuff!

[Power Surge]

Keep in mind that not all mechanical secondary Holleys are double pumpers. Some are mechanical secondary, with no accelerator pump. That is why the term "double pumper" is used; to differentiate between the two mechanical types.

[Bob In Ct]

Just for the record - An accelerator pump (or two) is used because when the throttle plates are opened quickly, air (being far less dense than gasoline) immediately enters the manifold. The gasoline (being more dense than air) takes a while to catch up following the usual carburator plumbing. The accelerator pump provides the gas necessary during that lull.

Without an accelerator pump, acceleration, especially heavy acceleration, would be proceeded by a big hesitation.

Bob

[Anthony]

Originally Posted by Power Surge

Keep in mind that not all mechanical secondary Holleys are double pumpers. Some are mechanical secondary, with no accelerator pump. That is why the term "double pumper" is used; to differentiate between the two mechanical types.

I have never heard of a mechanical secondary carb without an accelator pump on the secondary side. Can you show me a picture of one.

[Woodz428]

Originally Posted by Anthony

I have never heard of a mechanical secondary carb without an accelator pump on the secondary side. Can you show me a picture of one.

I would have to agree. They did make conversions back in the late 60s and early 70s, that I installed for a few people, it basically piggybacked the front nozzle with a longer set that fed through holes you would drill in the air horn. It used a gear drive to the secondary plate. But I don't recall them ever being offered by Holley on a carb. If they did, it was dropped when the double pumper came out.

[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.

[Power Surge]

Originally Posted by Anthony

I have never heard of a mechanical secondary carb without an accelator pump on the secondary side. Can you show me a picture of one.

Got a Jegs or Summit catalog? They're in there. Look for the mechanical Holleys with no secondary metering blocks.

Edit: Here's a pic...

[Anthony]

Originally Posted by Power Surge

Got a Jegs or Summit catalog? They're in there. Look for the mechanical Holleys with no secondary metering blocks.

I have looked through many catalogs over the last 20 years, as well as at car swap meets, and all of the double pumpers I have seen have had secondary metering blocks. Now, some of the vacuum secondary carbs have secondary metering plates, and some have metering blocks, but I have not seen any double pumpers with a metering plate.

I think an accelator pump, at least with a holley, reqiures the use of a metering block, whether it's the primary or seconndary.

[Power Surge]

Originally Posted by Anthony

I have looked through many catalogs over the last 20 years, as well as at car swap meets, and all of the double pumpers I have seen have had secondary metering blocks. Now, some of the vacuum secondary carbs have secondary metering plates, and some have metering blocks, but I have not seen any double pumpers with a metering plate.

I think an accelator pump, at least with a holley, reqiures the use of a metering block, whether it's the primary or seconndary.

That's because it's not a double pumper (isn't that what I said??). It's a mechanical secondary carb with with no secondary pump, and no jetted metering block.

[Anthony]

Originally Posted by Power Surge

Got a Jegs or Summit catalog? They're in there. Look for the mechanical Holleys with no secondary metering blocks.

Edit: Here's a pic...

I don't see an accelerator pump on the secondary.


What's the Holley list # ?

[Power Surge]

Originally Posted by Anthony

I don't see an accelerator pump on the secondary.


What's the Holley list # ?

Dude, are you mental or something???

[Anthony]

Originally Posted by Power Surge

Dude, are you mental or something???

Actually yes. Maybe I'll learn something here. The List # is a number that holley assigns to every model of their carburetors, and once you have that number, you can then identify the carb. I was going to look up that carb to see what its specs were.

[Power Surge]

Originally Posted by Anthony

Actually yes. Maybe I'll learn something here. The List # is a number that holley assigns to every model of their carburetors, and once you have that number, you can then identify the carb. I was going to look up that carb to see what its specs were.

Try 9776 and 4224. Not sure if those are the right list numbers, but pretty sure they are.

[392cobra]

9776 is a 450 cfm chokeless mechanical secondary with a secondary metering plate and no secondary pump.

I learned something new too.