BASIC CARBURETOR FUNCTION

IN THIS SECTION, YOU WILL FIND THE FOLLOWING SUBSECTIONS:

THE CARBURETOR
CARBURETORS
CONSTRUCTION OF A CARBURETOR MODEL
THE VENTURI AND THE THROTTLE PLATE
MAINTAINING A FUEL SUPPLY
CIRCULATING THE FUEL SUPPLY

THE CARBURETOR

The carburetor is the small engine component that mixes liquid fuel with air. When liquid gasoline is mixed with enough air to form a fine spray, the fuel becomes explosive and very easy to burn. The ideal mix of fuel and air for use in an engine is approximately one part gasoline to fifteen parts air. An air and fuel mixture that contains more fuel than this ideal amount is called rich, and an air and fuel mixture that has less fuel than the ideal is called lean.

A gravity feed carburetor. The term gravity feed indicates the way this carburetor draws fuel from the fuel tank. The fuel tank is positioned above the carburetor, and the force of gravity draws the fuel down through the fuel line into the carburetor’s float bowl. The fuel level in the bowl is controlled by a float that’s connected to a needle valve. As fuel enters the bowl, it causes the float to rise, closing the needle valve. When the needle valve closes, it cuts off the flow of fuel into the float bowl. When the float drops, the valve opens and allows more fuel into the bowl.

Fuel is drawn up out of the float bowl through the main fuel jet. The fuel jet is like a straw that sucks the fuel up out of the bowl. To explain how the fuel jet works, consider the example of a straw. Suppose you place a straw into a glass of water. If you blow air across the top end of the straw, the water level will rise inside the straw. This happened because as you blew across the straw, you created an area of low pressure on the column of air in the straw. Since the pressure in the straw was lower than the atmospheric pressure on the water in the glass, atmospheric pressure pushed the water up the straw.

If you were able to blow hard enough across the top of the straw, you could decrease the air pressure so much that the water would actually rise up to the top of the straw. A carburetor uses this principle to draw fuel up out of the float bowl.

The “straw” in the carburetor is the fuel jet. One end of the jet is in the fuel bowl and the other end of the jet is in the air passageway that leads to the engine’s intake. As air rushes over the jet, it creates a low pressure area that pulls fuel up the jet. However, in order to pull the fuel all the way up the jet, a device called a venture is used to speed up the air flow.

A closer view of the air flow through a carburetor. When a piston moves downward during the intake stage, the vacuum created in the cylinder sucks fuel and air into the cylinder through the intake manifold. The intake manifold is a metal pipe that leads from the carburetor to the engine cylinder. The central passage of the carburetor is called the throat. There is a rush of air as air flows through the carburetor throat.

The venture is the center area of the carburetor’s air passage that’s shaped like the center of an hourglass. As air rushes into the carburetor, it speeds up when it reaches the narrow venture. The end of the fuel jet is positioned in the venture area so that the high speed air passes right over it. The high speed air creates a low pressure area over the fuel jet, and gasoline is sucked up through the fuel jet. The air rushing past the fuel jet breaks the gasoline up into a fine misty spray. This spray of fuel is then drawn into the cylinder for burning.

The throttle is a round metal disc that’s positioned inside the intake manifold to control the flow of fuel and air to the cylinder. When you open the throttle, you let more fuel and air get to the cylinder, so the engine runs faster. When you close the throttle, you restrict the amount of fuel and air that gets to the cylinder, and the engine runs slower.

The choke plate is a device that’s placed in the carburetor throat to restrict the amount of air that flows through. When an engine is first started, it needs a rich mixture of fuel to get running. By closing the choke plate, less air flows through the carburetor, but the air flows faster. The faster air flow causes more gasoline to be sucked up through the fuel jet. Note that the choke plate doesn’t cut off the flow of air completely. The choke plate has a notch in it that allows some air to flow through it.

Some carburetors contain a rubber primer bulb in addition to a choke plate. Before the engine is started, the primer bulb is pressed with the fingers to pump gasoline into the cylinder through the carburetor. This action makes the engine start easier, usually on the first try. When no primer bulb is present, several pulls on the starter rope may be needed to start the engine.

CARBURETORS

Carburetor used to mix precise quantity fuel with incoming air. Carburetor constructed to maintain ideal ratio of about 14 to 1 (ideal ratio often listed as 14.7 parts air to 1 part fuel). Ideal ratio should be maintained from idle through maximum rpm.

CONSTRUCTION OF A CARBURETOR MODEL

Consider how model of carburetor can be built. Begin with 2 inch square block aluminum and aluminum cup 1 ½ inches in diameter by 1 inch high. Block will be drilled to form passageways, cup will be used to hold fuel.

THE VENTURI AND THE THROTTLE PLATE

Begin by machining passageways in block. First drill hole horizontally through block to serve as main carburetor passageway through which air will flow.

The venturi—next use specially shaped drill to increase size inlet and outlet areas. This makes passage hourglass shaped. Just created a venturi. Purpose hourglass shape to increase velocity of air as it passes through center of carburetor. Increased velocity causes air pressure to drop at center of hourglass. This lower pressure will cause fuel to be drawn from bowl into carburetor.

Can visualize action venture by using glass water and drinking straw. Place straw in water and blow across top of straw, will duplicate conditions in venture. Within narrow confines of straw, air pressure will drop. Atmospheric pressure on water outside straw higher than pressure inside straw, so water will push up into straw. If you stop blowing across top of straw, pressure will equalize, and water levels in straw and glass will even out.

The throttle plate—need method controlling air that flows through carburetor. Function provided by throttle plate. Drill hole through aluminum block perpendicular to air passage which crosses midpoint of venturi. Into this hole place piece copper or brass bar with two holes drilled into it to hold a disk of metal which when bar turned to move it across venturi, can block movement of air through passage.

MAINTAINING A FUEL SUPPLY

The fuel bowl—lower section carburetor needs to house fuel. Aluminum cup can be attached to aluminum block. To cup need to add fuel line made of plastic or rubber and two copper elbows which connect fuel line to inside of fuel bowl.

Another passageway also added in aluminum block to inside of venture. This the fuel bowl vent, necessary for two reasons. One so empty bowl can fill with fuel. If fuel enters bowl, bowl will not fill unless air present inside can be allowed to escape. Vent provides path through which air can leave bowl. Air and fuel vapors from bowl vented into carburetor so can be used by engine. Second reason for fuel bowl vent to allow atmospheric pressure to be exerted on surface of fuel in bowl. With carburetor, atmospheric pressure needed to force fuel up into venture.

The float and valve arrangement—to control level fuel in bowl, need valve system. Need float made of plastic, copper, or brass. Float will float on top of fuel in bowl. As fuel level drops, float will drop. Connected to float is needle valve (sometimes simply called needle). As float moves up and down, so does needle. Needle fits into machined area called seat. When float presses needle up into seat, fuel flow will be stopped. Once fuel in bowl dropped to certain level, needle released from seat and fuel will again flow. Float will rise once more to point where needle will stop flow.

CIRCULATING THE FUEL SUPPLY

The idle circuit—begin with idling. Idling refers to engine operating at lowest speed with no load. During idling, throttle plate normally in closed position, preventing large amounts of air from passing through venturi. If very little air flowing through venturi, how can enough air and fuel mixture be drawn into engine to make it operate? Answer is separate circuit built into carburetor to provide small amount fuel and air when engine idling. This circuit called the idle circuit.

Idle circuit is way for air to bypass closed throttle plates, mix with fuel from bowl, and enter engine. Idle circuit consists of several passageways drilled into carburetor. Idle air bleed extends from outside carburetor body to inside of throttle plate. It allows air to be brought into circuit. Another passage, called fuel feed tube or idle pickup, extends from inside float bowl to idle air bleed passage. Fuel feed tube allows small amounts fuel to be mixed with air being drawn into central passage of carburetor. Is an idle mixture screw which can inhibit amount of air being drawn into idle air bleed. This can control the relative amounts of fuel in the idle’s air fuel mix. Idle mixture screw can function in two ways, depending on design of carburetor. In some cases, screw used to control the amount of air flowing in idle circuit; on other carburetors, screw may be used to control amount of fuel mixed with incoming air.

Once air mixed with fuel, it’s discharged out into engine through small hole or slot located near edge of throttle plate. Idle discharge hole or slot located on engine side of throttle plate. In order for engine to receive flow, has to be some force that will pull air and fuel through idle circuit. Force is vacuum, area of low pressure created by engine.

As engine operates, air pulled into engine during intake stroke. As piston moves down on intake stroke, air drawn from carburetor into engine. If throttle plate opened wide, air drawn into engine with relative ease. When engine at idle, throttle plate closed, blocking airflow. Since airflow blocked, vacuum will form between engine and throttle plate. Because vacuum created on engine side of throttle plate, idle discharge hole must be located on same side of plate to operate properly. As long as throttle plate closed with engine running, air and fuel mixture pulled through idle circuit. Ratio of air to fuel can be controlled by idle mixture screw.

The intermediate circuit—look at what happens when throttle opens slightly. More air allowed to enter engine, therefore more fuel will be needed to mix with the air. Throttle not open enough to allow large amount of air to flow through venturi, so need another way to add fuel to mixture. Solution is to branch one or more passages off idle circuit. These additional passageways referred to as intermediate circuit.

Intermediate circuit consists of several discharge holes similar to idle discharge hole. However discharge holes for intermediate circuit located just behind throttle plate on air cleaner side of carburetor central passage. Some manufacturers may use slot rather than discharge holes. Since discharge holes of intermediate circuit aren’t located on engine side of throttle plate, no fuel mixture drawn through them when throttle plate closed or in idle position. As throttle plated opened slightly, these holes uncovered and exposed to engine vacuum. Once exposed, air and fuel will be drawn through them as well as through idle discharge hole. Additional fuel through intermediate circuit will maintain proper ratio in air fuel mixture.

The high speed circuit—airflow through venturi restricted as long as throttle plate partially closed. As a result, suction or vacuum created on engine side throttle plate. Vacuum a factor in operation idle and intermediate circuits. As throttle plate opens more, airflow less restricted. Without throttle plate blocking airflow through carburetor, air can flow easily into engine. Vacuum created by engine will actually decrease. When throttle plate wide open, is little or no vacuum created by engine. Therefore, idle and intermediate circuits provide no fuel. Additional circuit needed to supply fuel to engine, is referred to as the high speed circuit.

High speed circuit supplies fuel when throttle plate opened more than ¼ of way. High speed circuit operates on principal of venturi. The venturi a reduction in size of air inlet, which speeds up airflow and creates low pressure area at smallest point. Most venturi have hourglass shape which allows for smooth airflow. Typical carburetor will have passageway leads from fuel bowl up to venturi. This passageway referred to as fuel pickup tube or main fuel feed tube.

As air flows through venturi, air speed will increase and pressure decrease. Low pressure area in venturi creates suction effect that will draw fuel up pickup tube. Fuel leaves tube to mix with stream of air entering engine. The more air that flows through venturi, the more suction developed.

Exact amount fuel drawn out of pickup tube dependent on amount of air entering engine. This keeps consistent air fuel mixture. As throttle opens wider, more air allowed to enter engine. Because more air entering engine, need more fuel to maintain ideal ratio. As more air passes through venturi on way to engine, more fuel drawn up pickup tube. Action of venturi maintains consistent ratio of fuel to air. As throttle is closed and less air allowed to pass through venturi, less fuel drawn from tube.

As airflow changes under operating conditions, venturi keeps air and fuel consistent. Must be some way to establish within high speed circuit appropriate amount of fuel for typical mixture used in high speed circuit application. This usually accomplished by restricting size of inlet on fuel feed tube. This restriction that will control flow of fuel into tube commonly known as carburetor’s main jet. Small main jet allows only small amount of fuel to enter pickup tube while larger jet will allow more fuel to pass into engine. Sometimes carburetor will have adjustable main jet by which technician can adjust amount of fuel picked up. These types carburetors rarely used today. Modern carburetor typically has nonadjustable main jet, referred to as a fixed jet. Fixed jet restricts pickup tube’s inlet to an ideal size determined by manufacturer and isn’t adjustable.

High speed circuit will normally contain an air bleed passageway. Air bleed allows small amount air to mix with fuel in pickup tube. Small amount of additional air allows fuel to flow more easily out of venturi. Additional air also helps to vaporize fuel into mist that will burn better in engine than droplets raw gasoline. Inlet to air bleed passageway normally of fixed size to prevent excessive amount of air from mixing with fuel in pickup tube.