How Carburetors Work….

By Phillip Floria

The modern internal combustion engine is merely an air pump; it pumps air in and air out. One of the first things performance enthusiasts learn is to get more in you have to get more out. Performance exhaust is useless if you don’t increase the air/fuel mixture coming into the engine.

Ok you know that, so you also know bigger has got to be better, a bigger carburetor will allow more air to enter the engine along with a free breathing air box or air cleaner. This is true but only when other factors are taken into account like, jetting, and the type of cams used.

“Venturi Effect” The carburetor, which is basically a large tube or “Venturi”. Vacuum created by the downward stroke of the piston causes the higher pressure in the float chamber to force the fuel/air mixture through the fuel/air outlet passage (or ports) in the carburetor venturi and vaporized, by the fast moving air stream. A needle sits in the hole to regulate the fuel flow. The throttle cable or engine vacuum operates the needle up and down to regulate the fuel in the air stream.

Not all carburetors are as simple as the one above, some have more than one hole fuel to be drawn from. Most all-modern carburetors have an idle speed adjustment screw it is actually a valve; the end of the adjustment screw is shaped like a needle and it regulates fuel flow when the engine is running at low speed.

For higher speed operation a needle valve is attached to the slide, which is operated by vacuum or cable. In cases where the high-speed needle is attached to the slide, the slide is raised and lowered by the throttle cable. When the throttle is open the needle is raised up out of the emulsion tube allowing more fuel to mix with in increased air allowed by the rising slide. As the throttle is closed the slide and needle come down reducing air and fuel.

Most carburetors used on modern motorcycles also have a moving flap in the venturi to regulate the airflow, a butterfly throttle valve (or throttle plate). On most carburetors this is where the throttle cable is connected. As the throttle plate is opened it allows more air into the engine. Fuel is fed through a fixed orifice or jet controlling the amount of fuel in addition to the idle port.

Some of the more expensive aftermarket carburetors and most of the newer orignal equipment carburetors have additional circuits or passageways and ports for fuel to enter the air stream. In addition to the idle circuit, a transfer, midrange or air circuits may be used to fine-tune the fuel mixture. Remember the idle port will provide fuel during the entire operation of the carburetor. As the air speed increases the needle or main jet will add fuel in addition to the idle circuit.

The most modern carburetors are Vacuum controlled or CV (Constant Vacuum) type. A vacuum operated diaphragm controls the opening and closing of the slide (or piston). The CV Carburetor also has a butterfly throttle valve (or throttle plate).

At idle, with the throttle plate closed and the air stream cut off, idle speed is maintained by fuel metered through the slow jet. Air from the slow air jet mixes with the fuel and is delivered to the idle port at the vacuum side of the throttle plate. At low speed as the throttle plate is cracked open the transfer ports are exposed to the vacuum side of the throttle plate and additional fuel is directed to the barrel of the carburetor. With throttle plate cracked open a quantity of fuel also enters the air stream from the needle jet. The idle and transfer ports supply additional fuel to the carburetor barrel to assist during the transition period from idle to mid range.

The venturi opening is reduced by the low position of the vacuum piston (Slide). This enables initial air stream velocities to be higher than normally attainable with fixed venturi carburetors. The higher air stream velocities provide greater quantities of fuel necessary for good acceleration.

As the throttle plate is opened, airflow increases through the carburetor and the pressure drop in the venturi near the needle jet increases.


The low pressure in the venturi travels through the vacuum port in the vacuum piston to the chamber above the diaphragm. The chamber beneath the diaphragm is vented to atmospheric pressure by a passage from the chamber to the carburetor inlet. The higher pressure at the underside of the diaphragm overcomes spring pressure and moves the vacuum piston upward in proportion to the pressure difference between chambers.

As the throttle plate is opened further, the pressure difference between the chambers above and below the diaphragm increases and the vacuum piston moves further upward. The venturi opening increases and the needle is lifted further out of the needle jet. The quantity of fuel and the volume of air are simultaneously increased and metered to the proportions of engine demand by the variable venturi and needle lift.

The accelerator pump is used to supply fuel during sudden or rapid throttle openings. It quickly injects fuel into the venturi; the extra fuel provides for smooth acceleration and reducing throttle hesitation.

For cold starting carburetors are equipped with a choke or an enrichener circuit. The choke is a flap type valve similar to the butterfly or throttle plate. It is located on the inlet side of the carburetor and reduces airflow making the fuel mixture richer. As the engine warms up the choke is opened up and the carburetor returns to normal operation. An enrichener is a separate fuel circuit, manually controlled by a cable when it is engaged it provides additional fuel making the idle mixture richer.

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