Carburetor & Components of carburetor

Carburetor is a device which blends the air and fuel mixture for an internal combustion engine.

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Components of carburetor

The components present in the carburetor are float chamber, diaphragm chamber, venture, throttle, idle and off idle circuits, main circuit etc.


Float chamber:

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Float chamber is used to holds the fuel at the atmospheric pressure and needs to be ready for use. The fuel supply is refilled by a float driven controller. If the fuel level drop is observed in the chamber then it opens the inlet valve. This permits the fuel pump to transport extra fuel to the float chamber. After filling the fuel into the chamber, the level of the fuel must be raised by closing the inlet valve.

Diaphragm Chamber:

In any orientation the engine is operated and float chamber is not suitable. In place of the float chamber a diaphragm is used. A flexible diaphragm arrangement with individual side of the fuel chamber is arranged. Here the fuel is drawn out from the engine, with the help of the ambient force; the diaphragm is to be forced to inwards. To the needle valve the diaphragm is connected and is made to move inside. This opens the needle valve to allow more fuel, this helps in refilling the fuel as it is expand. If the fuel is refilled then the diaphragm moves out due to the pressure of the fuel and a minor spring, closing the valve of the needle. When it shows the steady state fuel reservoir level then it reaches the balanced state and it remains constant in any direction.

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Fixed venturi:

Fixed venturi is used to varying the air velocity, as the venturi alters the fuel flow. This construction is employed in most of the carburetors that was found on cars.

Variable venturi:

Variable venturi is used to inject the fuel jet that primarily differs by the slide. In the constant depression carburetors, the process is done by a void operated piston. The vacuum operate piston, is connected to a pointed needle which slides in to the fuel jet. A fundamental version exists that is commonly found on dirt bikes and motorcycles.  The needle and the slide must be directly controlled with the help of the throttle position.

Side draft SU carburetor is the most common type of the carburetor in the variable venturi. And the model is also similar for the zenith Stromberg, Hitachi and also few more additional makers.

The carburetor must be used under all engine working conditions:

Air flow of the engine must be measured

  • To keep the air mixture or fuel mixture in the proper range to deliver in the correct amount.
  • Mix the mixture evenly and finely

If the air mixture and the gasoline mixture are ideal fluids then the work is simple. Due to the viscosity, inertia and fluid drag are deviated from the ideal behavior. It requires a great deal of difficulty to pay for exceptionally for low or high engine speeds. A carburetor essentially provides the suitable air mixture and fuel mixture through a wide range of atmospheric pressure, ambient temperature, loads, centrifugal forces and engine speeds.

Modern carburetors are necessary to accomplish this task, by retaining low amounts of exhaust emissions.

Function properly under all the conditions, most of the carburetors hold a complex set of devices to support different operating modes called as circuits.

Main open throttle circuit:

Throttle is gradually opened; later the manifold vacuum is reduced. Hence, there is low limit on the air flow and the flow is to be reduced through off-idle and idle circuits. Due to Bernoulli’s principle the venturi profile of the carburetor throat originates to play (the pressure decreases and velocity must be increases due to the principle). The velocity of the air is to be raised as the high speed and low pressure sucks the fuel into the airstream over a nozzles or a nozzle. The nozzle is located at the center of the venturi. In some cases one or more added supporter ventures are located coaxially within the main venturi to increases the effect.

If the throttle is locked then the airflow through the venturi drops until the dropped pressure is insufficient to preserve this fuel flow and later the idle circuits takes over.

Velocity of the fluid is the principle of the Bernoulli’s theorem; it affects the large flow rates and large openings. The fluid flows at the small areas and a low speed is dominated by viscosity. The Bernoulli’s principle is unproductive at the slow running or idle in the small carburetors of the model engine. In medium size model engines they must have the flow restrictions of the jet aircraft to decrease the pressure sufficient to pull the fuel into the air flow. The slow and idle running jets of huge carburetors are located after the throttle valve somewhere, as the pressure is decreased partly by viscous drag, rather than by Bernoulli’s principle. The common rich mixture device to initiate cold engine was choking; it works on the similar principle.

Idle and off-idle circuits:

When the throttle closed or is nearly locked, then the manifold vacuum formed after the throttle is enough to pull a minor quantity of fuel and air through minor openings found next to the butterfly valve. This is known as the idle circuit. It allows the engine to running when there is not any pressure on the accelerator. The rotating valve moves forward to a little open throttle position, and then the vacuum is to be reduced. Some extra small beginnings are exposed to compensate for this, which is known as the off-idle circuit.

Main circuit:

If the throttle valve is gradually opened, then high amount of air is allowed through the pipe and also into the engine. The idle and idle-off engines are ceased to function as the manifold vacuum is now lowered. Through the venturi the air flow increases and the Bernoulli’s affects the lower pressure in the pipe and the velocity increases. It sucks the fuel into the air stream through a jet air craft into the center of the throat.

Accelerator pump:

If the throttle is unlocked (opened) very quickly, the idle circuit stops working immediately. The main circuit does not effective until the air flow has time to build sufficiently. To bridge the gap in the fuel flow an accelerator pump delivers a stream of fuel under low pressure to smooth the translation from idle circuit to main circuit.

The accelerator pump is also used mainly in the engine earlier, this is for the cold start. Excessive preparing like an improperly adjusted choke can cause flooding. This is when too much fuel and insufficient air are prevented to support combustion. So for this reason most carburetors are equipped with an unloader mechanism. The accelerator is fixed at wide open throttle while the engine is cranked. The unloader holds the choke open and admits the extra air and eventually the excess fuel is cleared out and the engine starts.


When it is cold then the fuel ignites less prepared and if the engine is also cold then fuel vapors can reduce out of the fuel air mixture onto consumption and cylinder walls. So this makes the mixture thinner. To compensate a valve known as choke is used. This controls the air flow at the entrance to carburetor, keeping the manifold low pressure even through the throttle valve that has been unlocked (opened).  Here, in this case fuel needs to be sucked into the incoming air through all the fuel circuit at once, even though they are idle or off-idle and main. In some engines instead of using a choke valve an added fuel behind the throttle valve can enrich the fuel air mixture.


The carburetor mainly consists of open pipe from side to side, where the air passes into the inlet multitudinous of the engine. The pipe looks like a venturi and it narrows at the section. It extends the pipe and it causes that the air flow to increases in speed through the tightest part. Below the venturi a butterfly valve is arranged which is known as throttle valve. Rotating circle that can be twisted ends onto the airflow, it barely restrict the flow at all points or it can be rotated where it completely blocks the air flow. By using the valve the air flow must be controlled in the carburetor throat and the quality of air or fuel mixture in the system will deliver in that way regulating the engine speed and power. With the help of the cable the throttle is connected, in other cases by using the mechanical linkages of joints, rods or by pneumatic link. To accelerator pedal on a car or the identical control on other equipment or vehicles this is used.

At the narrowest part one can observe the small holes from where the fuel is lead into the air stream through that holes of the venturi. At the other places the pressure is lowered and not running on filled throttle. By using the calibrated orifices the fuel flow must be adjusted and is referred as jets in the fuel path.

Power valve:

During the open of the throttle operation a rich mixture produces more power and also prevents the pre-ignition detonation. It keeps the engine cool. This is frequently addressed with a spring loaded power valve, which is held shut by engine vacuum. If the throttle valve opens up and the vacuum is to be decreased. With the spring the valve opens and more fuel enter into the main circuit. In case of the two stroke engines the operation of the power valve is reversed when compared to the normal engine. Generally it is in ‘ON’ condition and at a set RPM it must be turned to ‘OFF’. Basically it is activated at the high RPM to spread the engine revolution range, capitalizing on a 2 stroke tendency to revolution higher, for temporary situation when the mix is lean.

Alternate to hiring a power valve, the carburetor may utilize a step up rod system or metering rod to improve the fuel mixture under high demand conditions. Such systems were originated by carter carburetor for the primary two ventures of their four barrel carburetors. The step up rods are widely used for the most 1-, 2-, and 4- barrel carter carburetor through the end of production.

At the bottom end the step up rods are tapered and it extends to the main metering jets. To the vacuum piston the tops of the rod are connected. Mechanical linkages are used to lift the rods out of the main jets when the throttle is opened may or may not when the manifold vacuum drops.

If the step up rod is dropped into the main jet then it restricts the fuel flow. If the step up rod is raised up, from the main jet then more fuel can flow over it. In this way the fuel must be delivered is customizing to the transient demand of the engine. In case of 4 barrel carburetors they use metering rods only for the primary two ventures. In case of the Rochester Quadrajet both the primary and secondary circuits are used.

Working of the simple plain tube carburetor

A simple plain tube carburetor consists of two different suction chambers, one is known as the main unit and another one is known as float chamber.

The main unit is attached to the engine cylinder by means of pipe known as the intake manifold. When suction takes place inside the cylinder the vacuum produced in the cylinder extremes to the main unit through the intake manifold. This vacuum pulls the air from the atmosphere into the manifold. The area of cross section of the manifold keeps on decreasing in the direction of air flow at a particular point, the area of cross section minimizes in this condition. This section is known as venture or throat of the manifold, the velocity of the air at this point is maximizing. The fuel flows from the float chamber through a pipe to the main unit. The pipe is known as metering jet.  The tip of the metering jet ends in the main unit at the throat. The flow of the fuel through the metering jet is controlled by a screw attached to the other end of the metering jet. This screw is known as metering screw. The tip metering jet is the venture is known as the orifice or the discharge jet. The air and the fuel or both are present together in the main unit below the venture. A good mixing of air and fuel takes place in this space. Thus this space is known as mixing chamber.

During the mixing following operation are carried out:


It is the splitting of the fuel into very small and fine particles due to which the fuel spreads completely over the mixing chamber and very good mixing will takes place between the air and fuel.


It is atomization and a charge in the state of the fuel. The combustion process will be takes place efficiently inside of the cylinder due to vaporization.

Uniform distribution of fuel and air:

At all the points the fuel and the air ratio will be same in the mixing chamber. There is value below the mixing chamber known as throttle valve. This valve is controlled by the operation of a cable. The flow of air fuel mixture is controlled by the operation of the throttle valve.

There is a closed vessel that is floating on the surface of the fuel in the float chamber. This vessel is known as the float. A lever is attached to this float that operates a valve at other end is known as Needle valve.

Needle valve as the level of fuel decreases in the float chamber, the float moves down and the needle valve opens. Fuel from the fuel tank enters the float chamber through the needle valve as the level of fuel rises in the float chamber, the flow of fuel stops. Thus the level of fuel is maintained in the float chamber. This level is always 4 mm to 6 mm below the discharge jet of the main unit. There is a gap in the float chamber that is opened to the atmosphere. This gap is known as air vent atmospheric pressure which acts on the surface in the fc through the air vent. Here, the fuel flows through the metering jet with uniformity all time.

Types of carburetor

Carburetors are divided in to two types they are

  • Natural draft carburetor
  • Updraft carburetor
  • Downdraft carburetor
  • Float type carburetor

Natural draft carburetor

The natural draft carburetor is also known as side draft. If there is less space on the top of the engine at that case this type of carburetor is used. The air must enter horizontally into the manifold.

Updraft carburetor

In this type of systems gravity fed fuel supply is used. The carburetor is place above the fuel tank. By using the gravity the tank receives the fuel. In the engine the air fuel mixture must be forced upwards.

Downdraft carburetor:

The downdraft carburetors operate at the lower air velocities and larger passage. In this the gravity assists the air fuel mixture flow to the cylinder. This carburetor can deliver large measurements of fuel when required for high power output and high speed.

Float type carburetor:

The float is a small closed vessel made up of a plastic or brass. In this carburetor maintains a constant level of fuel in the float bowl. The float works much like one in a watering system, opening and closing a needle valve as the float lowers or raises.

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