To give you an idea how booster...
To give you an idea how booster shape can affect carb sizing, this is a Holley 750-cfm Dominator carb. Note how the diameter of the booster flares at the bottom. This makes the booster more sensitive, reduces the overall area of the venturi, and cuts the rated airflow through the carburetor by 300 cfm.
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.
 The fuel path for the main...  The fuel path for the main circuit begins in the float bowl (A), through the main jet (B), into the main well (C), and is emulsified with air that enters through the high-speed air-bleed (D) before arriving at the discharge point of the booster venturi (E). |  There are many different booster...  There are many different booster designs that have a dramatic effect on the shape of the fuel curve. The downleg booster places the booster closer to the narrowest portion of the venturi and is a more sensitive booster compared with a straight booster used on a 0-1850 600-cfm carb. |  Venturi Effect High Pressure...  Venturi Effect
High Pressure Low Velocity
Low Velocity High Pressure
High Pressure Low Velocity
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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.
This is an emulsion tube found...
This is an emulsion tube found in most Holley carburetors. This tube sits in the main well and mixes the fuel with air from the air-bleed and adjacent air channel that eventually finds its way to the venturi booster.
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.