Top End

A carburetor mixes fuel and air in the right proportions and delivers it to the engine. It also atomizes the fuel, turning the liquid gas in the carburetor into a gaseous mixture with the fuel dissolved in air.

The main parts of the carburetor are the bowl, the jets, and the venturi (the open tube that goes through the center of the carb). The bowl is what holds the fuel. The jets are tubes that allow fuel to flow up from the bowl into the venturi. The venturi is a hollow tube running the length of the carburetor and attached to the head of the engine.

The basic process in a carburetor is as follows:

• air flows through the venturi and into the head
• the motion of the air creates a negative pressure in the venturi
• the negative pressure exerts a force on the fuel in the jets
• the fuel is sucked up towards the venturi
• en route, it is mixed with air in the jets and atomized
• the fuel further mixes with air upon entering the venturi
• the entire mixture is sucked into the head to be burned

Different controls are used to achieve the optimal fuel:air ratio for given throttle openings and to allow the engine to run at idle.

An air : fuel ratio that is lower than the desired ratio is termed “rich”, while one that is higher than the desired ratio is termed “lean.”

At different throttle openings, the engine also requires different absolute amounts of fuel. This is accomplished using different fuel systems.

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The throttle valve has a triangular section removed facing the intake side called the cutaway. The cutaway helps to create a negative pressure in the venturi because air hitting the valve at the cutaway is forced down and under the valve. This forces the air to move faster (think of a river that narrows – the water has to move faster) and creates a negative pressure in the venturi. A greater cutaway creates a lower pressure in the venturi, and causes more fuel to be sucked up through the jets. Similarly, a smaller cutaway creates a relatively higher pressure in the venturi and less fuel is sucked up.

Tuning the throttle valve has the greatest effect on the engine between approximately 1/8 throttle and 1/2 throttle.

The idle adjusting screw only affects the fuel flow to the engine at idle.

The pilot jet and air screw are tuned in conjunction to provide the optimal air : fuel ratio at idle. They have the greatest effect on fuel flow between idle and 1/4 throttle.

The needle jet, like the pilot jet, allows fuel to be drawn from the bowl into the venturi. It is a narrow jet that sits inside the main jet. Air drawn through the needle jet is metered with air from the air jet, allowing the fuel to be atomized, before entering the venturi.

The jet needle is a needle attached to the bottom of the throttle valve that moves in and out of the needle jet as the throttle valve moves up and down (as the throttle is opened and closed). The needle is tapered such that, when the throttle is closed, it blocks most of the area of the needle jet opening and very little fuel in the needle jet is exposed to the low pressure in the venturi. As the throttle is opened and the needle is pulled out, the taper causes it to occupy less of the needle jet opening area, allowing more fuel to be exposed to the low pressure in the venturi.

The low pressure in the venturi can be described as an attractive force being exerted equally over the entire surface of any fluid at a higher pressure (ie the fuel/air in the jets/bowl) because P = F / A (pressure equals force per unit area). For a given throttle opening, the negative pressure (and thus the force associated with it) is constant. The amount of fuel that is drawn is thus dependent on the surface area of fluid (gas/fuel mixture in the jets) exposed to the negative pressure.

The jet needle is designed to control the amount of surface area of fuel mixture exposed to the negative pressure of the venturi for any given throttle opening. As the throttle is opened, the jet needle is tapered so as to occupy less surface area and allow more fuel to be drawn.

Around 15% throttle, fuel starts being drawn from the needle jet. Above 3/4 throttle, the fuel flow is mainly controlled by the main jet because the jet needle is pulled high enough to essentially not block any of the surface area of the jets. Adjusting the needle jet and jet needle is thus most effective for 1/8 to 3/4 throttle.

Adjustment of the main jet is most effective between 3/4 and full throttle.

The air jet is a small opening on the intake side of the carburetor. Air flows through the air screw into the needle/main jets where it is mixed with fuel and drawn into the venturi.

Adjustment of the air jet is most effective between 1/2 to full throttle.

The floats maintain a constant level of fuel in the bowl of the carburetor. If the fuel level is too high, too much fuel will be drawn into the venturi, and vice versa if the fuel level is too low. Floats move up and down on a metal shaft and are made of a material that floats on gasoline. As the fuel level in the bowl increases, the floats move up. Eventually, the force associated with the pressure from the intake valve matches the force of buoyancy on the floats, and fuel stops flowing. This constant level of fuel is thus maintained in the bowl.

The floats are carefully manufactured to maintain the desired level of fuel in the carburetor, and thus shouldn’t be tampered with or adjusted.

Various systems are employed in carburetors to provide the rich (low air : fuel ratio) mixture required to start the engine.

In a starter plunger type carburetor, a plunger on the outside of the carburetor can be pushed to deliver a rich mixture to the engine. In this system, the throttle must be closed in order to create the vacuum necessary to draw fuel into the plunger tube.

A carburetor with a choke allows you to close the choke and limit air intake through the venturi, thus richening the mixture.

Finally, some carburetors have a fuel enrichening system which has a lever on the exterior of the carburetor that opens a different set of jets and allows more fuel to flow.