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Circuit City

How A Carburetor Works

Jeff Smith Jun 1, 2002

Step By Step

This is the float circuit. The reservoir of fuel sits in this chamber and feeds all the other circuits in the carburetor. It’s important to remember that as the float level rises, the overall fuel curve becomes richer. Accordingly, as the float level falls, the fuel curve becomes leaner.

The float circuit is very simple. As the fuel level increases, the float rises and eventually closes a simple needle-and-seat that prevents additional fuel from entering the float until the level drops.

The idle circuit offers tremendous opportunity for improved driveability. Note that the off-idle transfer slot distributes fuel before the idle-mixture screw. Therefore, the idle-feed restrictor determines the volume of fuel delivered at the transfer slot, not the idle-mixture screw.

All carburetors work on Bernoulli’s principal that high velocity in the venturi creates low pressure. This low-pressure area initiates fuel flow when high atmospheric pressure in the float bowl pushes fuel into the venturi.

These are the main jets located at the bottom of either the primary or secondary metering block of a Holley four-barrel carburetor. These replaceable jets are the restrictors that determine the air/fuel ratio at part- and wide-open throttle.

The low-pressure area created by air rushing past the booster venturi initiates fuel flow from the float bowl through the main jet. The main jet size determines the amount of fuel delivered to the boosters and into the engine. The pressure in the narrow point of the venturi is lower than the air pressure above the venturi. When this occurs, atmospheric pressure pushes fuel from the float chamber through the jets and into the engine.

The power-valve circuit adds fuel when demand is the greatest. At part-throttle, main jets can be sized for part-throttle economy instead of WOT power. This circuit is most often used only on the primary side of a four-barrel carburetor.

The accelerator-pump circuit adds fuel when abrupt throttle-position changes demand additional fuel to cover up a loss of signal to the main circuit. Without an accelerator-pump circuit, the engine would hesitate whenever quick changes in throttle position occurred.

Drawings are good for understanding basic circuits, but it’s also a good idea to know how all those basic circuits work in a Holley. This is the accelerator pump outlet or “squirter” on a Holley that shoot fuel into the primary bores. Also note the idle air bleeds (1) and the high-speed air bleeds (2) located adjacent to the squirter.

We can’t possibly cover all the details of a Holley carburetor in one short story. Dave Emanuel’s Holley carburetor book goes into much more detail and is worth the investment.

So you think you know how a carburetor works? Let's find out. Can you detail the major players in the idle circuit? What part does the power valve channel restrictor play in the main metering circuit? If you can answer these questions completely, then you win a gold star. Unfortunately, while almost everyone knows what a carburetor is, not everyone knows how it works.

This story is intended to unmask the mystery around the five basic circuits in a carburetor. Once you understand how these circuits work and how they interact with each other, then you will know how to better diagnose and modify your carburetor to make your engine run better and more efficiently.

Five Circuits We'll focus this discussion on a typical Holley carburetor. In terms of the basic circuits, all carburetors, not just Holleys, utilize the same techniques. The five most basic circuits in the carburetor are the float, idle, main-metering, power-enrichment, and the accelerator-pump circuits. There are other circuits as well, such as the choke and a vacuum-secondary system, but we'll concentrate on the most important five.

Float Your Boat

The simplest carburetor draws fuel from a small reservoir known as the float bowl. This system uses a float, a lever arm, and a needle-and-seat to regulate the float height. This works just like a float system in a toilet. Fuel enters through the carb inlet past the needle-and-seat when the float level is low. The fuel then fills the float bowl until the float rises sufficiently to close the needle against the seat. At this point, fuel stops entering the bowl until the float drops enough again to pull the needle off the seat.

The most important point in regards to the float circuit is that fuel height directly affects the air/fuel ratio. As the float level rises, this creates more hydrostatic pressure on the jets, which means increased fuel flow even with no other changes. This affects both the idle circuit and the main metering circuit.

Idle Thoughts

The idle circuit is fairly simple, yet has a dramatic effect on street manners, idle quality, emissions, and fuel mileage. Fuel enters the idle circuit through the main jet and then travels through an idle-feed restrictor and into the idle-feed well. Once in the idle well, atmospheric pressure pushes the fuel up across an idle air bleed located normally at the top of the barrel. This air bleed mixes air with the fuel, and then this mixture is pushed down an adjacent passage that leads past the idle mixture screw and then out the curb-idle discharge port located underneath the throttle blades.

There's also an idle transfer slot outlet that's located just upstream of the idle- discharge port. This transfer slot discharges additional idle fuel once an opening throttle blade uncovers the slot. This feature introduces additional fuel into the airstream as the throttle is opened, which prevents a lean hesitation that can occur due to an as-yet-inactive main metering circuit. This transition fueling occurs in the first few degrees of throttle opening.

The Main Squeeze

A carburetor operates exclusively on pressure differentials. Bernoulli's law, named after an 18th century physicist named Daniel Bernoulli, states that pressure is inversely proportional to velocity. This means that as air velocity increases, it decreases in pressure. This is also called the venturi effect. In a carburetor, the venturi is the restrictor, or smallest area of the barrel. As air flows through this smaller area, it increases in speed and simultaneously decreases in pressure.

When you connect the float bowl to the venturi area, the higher atmospheric pressure in the float chamber pushes fuel toward the low-pressure area in the venturi. Contrary to popular belief, a carburetor does not suck fuel into a venturi. Instead, atmospheric pressure acting on the fuel in the float bowl pushes fuel toward an area of low pressure. The greater the difference in pressure, the harder fuel is pushed into the area of low pressure. Put a fuel restrictor (called a jet) in the fuel port, and you have the main metering circuit.

The main metering circuit consists of a jet that restricts the total amount of fuel introduced into the main metering well. Inside the well is a small tube with even smaller holes drilled in it. This emulsion tube is designed to mix air with the fuel. The air comes from a high-speed air bleed located at the top of the idle well. This air bleed acts as both an emulsifier and a suction break to prevent fuel from siphoning into the engine after the engine is shut off. The air and fuel is then directed to a discharge nozzle that's part of the booster venturi located in the throttle-bore airstream. As the throttle is opened, air moves past the booster venturi, creating a low-pressure area that pushes fuel through the main circuit into the airstream. The high-speed air bleed contributes additional air into the main well to improve fuel atomization and circuit activity.

More Power

A carburetor doesn't really need a power-enrichment circuit. However, the main metering system is required to supply fuel to the engine both at maximum demand under wide open throttle (WOT), and also at part-throttle. >> WOT metering, dictated by the jet size and other variables, determines the amount of fuel necessary to maintain the proper air/fuel ratio at this maximum demand. But this means that at part-throttle when fuel demand is reduced, the jet will flow more than necessary.

The power circuit creates an on-demand enrichment circuit that adds more fuel during WOT operation. This circuit uses manifold vacuum to keep a small valve closed during part-throttle operation. When the throttles are opened far enough to lower the manifold vacuum to a certain level, a small spring in the valve opens the valve and additional fuel is introduced into the main metering circuit. All carburetors employ a power circuit, but the way they make this work is slightly different.

Accelerate It

The accelerator-pump circuit is also a power-enrichment circuit, but operates slightly different. If the throttle were always operated very slowly, you probably wouldn't need an accelerator-pump circuit. But hot rodders like to mash the throttle down very quickly. This creates an instantaneous and short-term loss in velocity in the system. Since that lower velocity means a lower pressure differential between the booster and the float bowl, this will create a lean condition or a bog.

To cover up for this short-term loss in fuel enrichment, all carburetors use an accelerator-pump circuit that shoots fuel directly into the venturi to prevent this hesitation. The circuit uses a small diaphragm or piston that is operated with a lever located on the primary throttle shaft. Add all these circuits together and you have the basis for a working carburetor. There's much more to learn about carburetors than we can cover in a few short pages, but the information is here if you're looking to be a little smarter than your friends.


Holley Performance Products
Bowling Green, KY 42101

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