Car Craft
Click here to find out more!

The Ultimate Guide to Holley Carburetors

Everybody has a least carburetor, but not everybody knows how to tune them, so we're giving you a bunch of charts, graphs, advice and tips so you'll know just about everything you'll need to get your carburetor tuned right.

Photography by

Main Circuits-or a Trip Down Venturi Highway
If there is one part in the world of high performance that everybody knows, it has to be the Holley four-barrel carburetor. The Holley has evolved through hundreds of variations and dozens of popular models, but the basic four-barrel carburetor has changed very little in the last 50 years. This is a great advantage for car crafters because if you're armed with the basics on one carb, that knowledge will carry you through all the different variations. We even have some new stuff for those of you who think you've seen it all.

While everybody thinks they know a lot about carburetors, there's always more to learn about even basic fuel circuits. We'll concentrate on the most important one: the main metering circuit. This circuit is simple when you break it down to its essentials. Let's take a look.

Fuel enters the float bowl where the height of this reservoir of fuel is regulated by the float that controls the needle and seat. Near the bottom of the bowl is a pair of main metering jets that restrict the amount of fuel that enters the main metering system. After passing through the jets, fuel collects in the main metering well at the same height as the fuel in the bowl. Extending into this main metering well from the top of the metering block is an emulsion tube. This tube is drilled with several holes intersecting the tube at various heights. These holes are used to mix air with fuel. The air is introduced through an air-bleed located at the top of the carburetor usually near the entry to the venturi. This is most often referred to as the high-speed air-bleed. Additional air is mixed with the fuel through emulsion holes in an adjacent air chamber. This air/fuel mix travels across a short passageway and exits from the booster located in the main venturi.

It requires a small amount of force to push fuel literally uphill in this circuit. As air speed increases through the main venturi of a carburetor, it creates an amplified low-pressure area inside the booster venturi. Because atmospheric pressure pushing on the fuel level in the float bowl is greater than at the booster, the pressure difference is enough to push the fuel through all the restrictions and passageways on its way out the booster and into the intake manifold.

The key to a properly designed performance carburetor is to move fuel out of the booster as efficiently as possible while creating the proper air/fuel ratio as the engine runs through its rpm band from just past idle to redline on the tach. This is the job of the main jet, the emulsion holes in the main well, the high-speed air-bleed, and the shape and design of the booster venturi, plus a bunch of other obscure variables. All these components work together to create the fuel curve that helps make horsepower.

Emulsion Immersion
You'll see references here to emulsion circuits, an important-sounding term that describes mixing fuel with air. Think of it as blowing bubbles in the fuel. All carburetors mix air with fuel in the venturi just before it enters the engine. But carburetors also mix air with fuel farther upstream inside the carburetor in the main well to make the liquid fuel easier to manage. Holley carburetors use a parallel air well that transfers air into the main well through two, three, and sometimes five holes that can be seen in the metering block photo (page 36). While this passage is called an air chamber, fuel still resides in this chamber at the same level as the fuel in the float bowl. As fuel demand increases at higher engine speeds, the float level drops, uncovering the lower holes in the emulsion circuit, which adds more air and leans out the air/fuel ratio. Combining the main jet flow area with the area of these emulsion holes and the high-speed air-bleed (along with a raft of other minor inputs) creates the basic fuel curve.

Enjoyed this Post? Subscribe to our RSS Feed, or use your favorite social media to recommend us to friends and colleagues!
25 comments
Michael Smith
Michael Smith

Lmao, what the hell is a power jet? Don't blame the carb for your poor tuning.

Nora Beldo
Nora Beldo

Nelson Malick so this is it?now I get it..

Hans-Jörg Ramforth
Hans-Jörg Ramforth

@ Russel: that what I had thought too for a long time. And also I worried about the PV, until someone told me about 2-step PV's. And now there is only one way to find out ;-) . Doing a self compare test. That's what I am going to do. The oponents are Carter 625cfm against Holley. the reason I ask for books is I live in Germany and have to learn Holley by myself, as I did with my Carter :-)

Page Crow
Page Crow

There will always be both types of induction for the masses. Some will choose EFI for all the right reasons. Some will choose the carb, for simplicity, reliability & beauty.

Page Crow
Page Crow

Yes. Rebuilding Holleys is easy. It's like building a model car. Lots of little parts. Take them apart. Clean them. Put new parts in. Put it back together. It's fun.

Page Crow
Page Crow

Read this article, for starters. The more you read, the easier it gets. Holleys are easy & cheap and work great.

Keith Ferch
Keith Ferch

My dad bought a holly little over 20 years ago its had a couple rebuilds and havent had no problems with awsome carb

Greg Vanscoyk
Greg Vanscoyk

don't buy a holley they always blow power jets

Murray P Cakes
Murray P Cakes

As the quantity sold goes up the price of TBFI will go down and carbs will become obsolete sooner than we think.

Russell Irwin
Russell Irwin

Holley is good for straight flat lines. Edelbrock is the way to go for street.

Michael Smith
Michael Smith

You guys had an article back in like 02 or 03 that really taught me how to tune a Holley. I still use it to this day.

Rich Sindora
Rich Sindora

Still the best carburetor ever for power.

Hans-Jörg Ramforth
Hans-Jörg Ramforth

Hi, I am interested in Holley Carbs. Do you have any book recommendation for me? Appreciate you support. Thx

Karl Loper
Karl Loper

I think you have to try to screw up a Holley tune!

Car Craft