What are Carburetors? Definition, Parts, Diagrams, How it Works, Types

Carburetors, the heart of automobiles, are described along with its definition, parts, diagrams, how carburetor works along with its various types. We all know vehicles need to run on combustion principles.

It needs fuel gasoline to be mixed with correct proportions of air to cause combustion. This combustion helps to generate the power to move the vehicle.

What are Carburetors? Definition, Function

So how the air-fuel mixture is managed? Here comes the carburetor’s role for managing the air-fuel mixtures for correct quantity.

Though these are not found in modern cars but they were used in almost every vehicle type from commercial to racing. The main goal of the carburetor is to mix just the right amount of air-fuel mixture to get the desired and optimum combustion possible.

carburetor definition basic
Carburetor definition basic

Carburetors Definition

We all know that in the case of gasoline or petrol engine, fuel is used and with the help of spark ignition along with air, combustion happens in the combustion chamber. The component which is used to mix the fuel with air for combustion is defined as a carburetor.

  • It is applicable only in internal combustion engine.
  • It is applicable only in petrol engine or where spark ignition is applied.

Simply, the carburetor supplies fuel for the combustion. It mixes with the air and fuel mixtures at the correct quantity needed for optimum combustion processes.

Stoichiometric Ratio

The optimum air-fuel mixtures are known as stoichiometric ratios. It’s the ideal ratio of air-fuel mixture which is 14.7:1.

It denotes for every 1 gram of fuel you need about 14.7 grams of air for ideal combustion.

That’s where the role of carburetors role kicks in.

Carburetors History

The invention of carburetors was going through a very big cycle, let us glance in brief,

  • If we look towards history, the first carburetor was invented by Samuel Morey in 1826.
  • Later in 1872, Siegfried Marcus filed a patent to use carburetor in petroleum engines. That was later developed by Carl Benz.
  • The use of carburetor was more popular in 1980s vehicles in the USA. Cars like Subaru Justy, Mazda B2200, Jeep Wagoneer were all based on Carburetor in those times. But they were phased out mostly after the 1990s due to the introduction of an efficient fuel injection system.
  • But these were still used in stock car racing events like NASCAR in 2011.

Let’s know more about the functions and the parts of the carburetor.

Functions of Carburetors

  • The main or prime function of a carburetor is to supply the air-fuel mixture in an optimum ratio to get the highest combustion efficiency.
  • Regulation of air-fuel mixture ratio.
  • Varies amount of mixture according to the engine speed and load changes.
  • Supply the correct amount of air-fuel mixture at various loads and speeds.
  • It helps to control the engine speed by regulating the air-fuel mixture.
  • It helps to properly burn the fuel.

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Parts of Carburetors

So, what are the parts of the simple carburetors? Let’s try to understand. There are various parts of carburetor.

It has the followings,

  • Carburetor Venturi
  • Throttle Valve
  • Strainer
  • Idle System
  • Off idle or part idle circuit
  • Open throttle circuit
  • The metering system
  • Float chamber and mixing chamber
  • Accelerator Pump
  • Carburetor Choke Valve

Diagram of Carburetors

A simple carburetor diagram is indicated below, so have a basic idea before the description of carburetor parts.

carburetor diagram
Carburetors diagram

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Carburetors Parts Description

Let’s check the components one by one.

Carburetor Venturi

What is Carburetor Venturi?

We have already learned venturi meter, and here, carburetor venturi also works based on the same philosophy. It is basically a passage that has a gradual decrease in its cross-section area.

Function & Working Process

This venturi is narrow in the middle and air is passed through this narrow tube. Airflow is the same within this tube and air velocity is varied based on the cross-sectional area. Air velocity is maximum in the narrowest section.

It reduces pressure and creates slightly low pressure.

Based on the variation of the cross-sectional area, air quantity is adjusted in the mixture.

Based on the shape or function, carburetor venture can be many types,

  • Plain or Single Carburetor Venturi
  • Double Carburetor Venturi
  • Triple Carburetor Venturi
  • Vane Carburetor Venturi
  • Nozzle Bar Carburetor Venturi
  • Variable Carburetor Venturi

Check out a very basic understanding of a simple carburetor, by ADTW [a door through a window],

Throttle Valve

What is Throttle Valve in Carburetor?

The throttle valve is a major part of the carburation system. We have already learned that there is carburetor venturi is to adjust the airflow. Below the venturi, there is a butterfly valve known as the throttle valve.


It just acts as our home water tap. Just like the water tap the flow can be controlled as fully open or close same happens in case of the throttle valve. It controls the air which can be passed to mix with the fuel.


The throttle valve link is connected to the accelerator of the vehicle.

  • When the driver increases the speed through accelerator, the air will be taken more and vice versa.
  • On the other side the fuel is taken into the air stream through small holes at narrow part of the venturi.
  • Whereas at other places pressure is lowered when not running at full throttle.



The strainer is filtering equipment used to filter the fuel.


The function of the strainer is to filtrate. Here, it filters the fuel before entering the float chamber.

How Does Strainer Works in Carburetors?

It has a wire mesh structure that is responsible for removing dust and suspended particles. If these particles are not removed it can serious blockage issues and the vehicle will not start.

Idle System

When the engine is going to be in idle condition, the throttle valve will be almost closed. Airflow through will not be able to create enough vacuum in the carburetor venturi which will not be able to pull the fuel.

In this case, the idle system comes into the picture. This system or circuit helps to feed fuel into the air.

Normally, at below 800 rpm, the carburetor idle system provides the air-fuel mixture.

It consists,

  • Main discharge tube,
  • Idle passage
  • Bypass
  • Low- speed jet,
  • Idle air bleed,
  • Economizer,
  • Idle screw port, &
  • Idle mixture screw.

Off Idle System

What is the Off Idle system in Carburetor?

Off idle or sometimes called part throttle is the main part used to feed more fuel. When the throttle valve is slightly open from the closed position, fuel delivery holes are uncovered.

The idle system alone is not able to meet the fuel requirements in the air stream of the carburetor. When the engine changes from idle condition to high speed, the off idle system is able to supply the fuel.


The low-pressure area is created by the throttle valve which blocks the airflow. Thus, allowing more fuel to enter to compensate for reduced vacuum pressure and to smooth the operation to a regular open circuit.

  • It is an idle system extension.
  • It works above 800 rpm.
  • If off idle system is not present, fuel mixture will be lean mixture.

The idle system will not be able to supply a sufficient amount of fuel alone, and an off idle system is required.

It supplies fuel when the vehicle changes its idle condition to high speed.


The operation of the Off idle system, as follows:

  • The driver of the vehicle presses down the accelerator.
  • The throttle plate will be slightly opened.
  • Off idle ports will be exposed towards the manifold vacuum of the intake.
  • This vacuum pulls the fuel out of the idle screw & the off idle port.
  • Provides sufficient extra fuel to mix with the additional air.

Open throttle circuit

What is Open throttle circuit in carburetor?

When the throttle valve is getting slightly opened the manifold vacuum gets weakened because of less restriction to airflow. This results in reducing the fuel flow through off-idle circuits. Now the role of Venturi components kicks in.


We know Bernoulli’s principle, where the velocity increases pressure will fall.

  • The venturi increases the velocity of air causing the causing the drop in pressure fuel. Hence, the fuel gets sucked through the nozzles located in the centre of venturi.
  • Whereas, when the throttle valve is closed again airflow will drop till the pressure where the pressure is not capable to maintain the flow.

The metering system

As the name suggests it controls the flow of fuel into the nozzle. It is responsible to get the correct mixture of air and fuel.

It has two parts,

  • Metering orifice and
  • Fuel discharge nozzle.

As we have already seen low pressure is generated when the air passes through the venturi. Due to this fuel gets discharged into the air stream.

The main function of the metering system shall be as follows,

  • Maintaining air-fuel proportion in mixture.
  • Reduction of pressure at the discharge nozzle.
  • It controls the rate of air flow at full throttle

Hence, the quantity of fuel is controlled by the metering orifice and discharge hole at the exit of the nozzle.

Float chamber and mixing chamber


This is for the storage of the fuel so as to maintain the continuous supply of the fuel.

  • If the fuel gets less than required amount, then the float will move downward causing the fuel supply valve to open and admit the fuel.
  • Similarly, when fuel supply is more float goes upward restricting the fuel admission.


  • This is where the actual mixing air and fuel is obtained.
  • The mixture of air + fuel is created in this chamber and then supplied to the engine cylinder.

To avoid the heating of the fuel in the float chamber in hot climates the heat deflectors and insulating gaskets are used.

It reduces the “heat soak” effect.

Accelerator Pump


Gasoline is denser than air, so obviously, air will react more and travel quickly compared to Gasoline. When the driver accelerates suddenly the throttle valve opens up immediately tries to admit the mixture even more.

carburetors accelerator pump
Carburetors accelerator pump

But the air pressure inside the manifold increases which decreases the evaporation of the fuel, this results in less fuel gets entered into the engine.

When the quantity of air is more the mixture is called a lean mixture. When the too lean mixture is transferred to the engine, the engine causes misfiring.


All these problems are corrected by the use Accelerator pump.

  • It forces the small amount gasoline quantity through a jet into carburettor throat.
  • So this extra fuel will counter the lean mixture conditions and will adjust the mixture suitable for combustion.

Choke Valve


The choke valve is most useful in cold climatic conditions. In cold conditions, the fuel is present in less quantity in the cylinders and makes difficult the starting of the engine. So to counter this situation more rich mixture needs to be provided.

In short, more fuel should be supplied compared to the air to start the engine. So the choke controls the flow of air at the entrance of the carburetor.


You may have tried opening up the choke valve mostly on two-wheeler bikes.

  • The choke restricts the air flow which results in more admission of fuel compared to air.
  • In case of cars there was manual chokes available but later automatic chokes were employed. But there is one thing to note that, we should deactivate the choke once the engine started.
  • If the choke is not deactivated it will cause more fuel admission, increasing emissions.
  • The fuel will be wasted and vehicle may get other problems.

Parts of Modern Carburetors

Since there are many disadvantages of the simple carburetor, it is modernized and added a few additional modern components, like

  • Automatic mixture control
  • Throttle return check
  • Anti-dieseling solenoid

Automatic Mixture Control

Automatic mixture control means a plunger-shaped valve, which is operated by a solenoid and Spring.

  • Automatic mixture control system controls the separate jet in the float chamber.
  • Valve is open when the solenoid is turned on. It increases the fuel supply to the jet.
  • The solenoid is operated by the signals from engine speed etc., and everything is measured by an electronic device.
  • If the solenoid is turned off, the spring helps to close the valve to decrease the fuel supply.

Throttle Return Check

When the throttle valve is open completely, the engine will run at a very high speed. It may create a high intake manifold vacuum.

  • This vacuum may have a probability to draw exhaust into the engine intake.
  • This exhaust will dilute the intake proportions.
  • Misfiring or even stall may happen.

Now, to avert this problem, the throttle return check valve is connected in modern Carburetors with the help of linkages.

Anti-dieseling Solenoid

Now a day, vehicles are used various modern emission control engines. These engines are normally working at high temperatures.

  • Due to this high temperature, there may be a chance of producing hot spots in the combustion chamber.
  • Hot spots can create pre-ignition in the combustion chamber.

To avert this pre-ignition, a modern carburetor is provided with an anti-dieseling solenoid and turns off the pre-ignition. Now we have seen all the parts present in the basic carburetor structure.

There are some additions to the newest carburetors like throttle return check, anti-dieseling solenoid, and automatic mixture controls.

Now let’s see how the Carburetor works.

How does carburetor works?

The structure of the carburetor may seem a bit complex but the working is pretty much easy to understand.

how does carburetor works
How does carburetor works

Let’s know the working pointwise to understand it easily.

  • The air flows down the pipe and it passes through the narrow venturi as we discussed in the above sections. Due to a decrease in the pressure, the sucking effect is created.
  • This sucking effect causes the fuel admission and the air-fuel gets joined for further process.
  • At the start of the vehicle, the choke can be turned on so as to start the engine in cold regions in rich mixtures.
  • The driver accelerates the link is connected to the throttle valve. The more throttle gets opened the more air will be admitted and similarly the fuel.
  • As the accelerator is turned more, a large quantity of air fuel mixture is admitted, hence the engine generates more power and the vehicle speeds up.
  • The fuel chamber supplies the fuel. When the fuel goes down float will move down causing more fuel to admit. If the fuel is more the float will close the valve restricting the admission of fuel.

This cycle runs continuously till the vehicle is switched on. There are various types of carburetors used let’s know more about them they are very easy to understand.

Types of Carburetors

There are three types of carburetors namely,

  • Updraft type carburetor
  • Horizontal type carburetor
  • Down-draft type carburettors.

These types are differentiated on the basis of air intake.

updraft carburetors
Updraft carburetors

Updraft Carburetors

  • The updraft carburetors are those in which the air is supplied from the bottom of the mixing chamber.
  • This one is placed lower side of the engine and uses gravity fed-fuel supply. In simple words, the tank is placed above the carburetor and fuel falls into it.
  • The air-fuel mixture is forced upward.

Horizontal Carburetors

  • The horizontal carburetors get the air from one side of the carburettor. Hence it is known as horizontal carburetors.
  • Basically, if you rotate the downward draft carburetor it will be known as the horizontal carburetor.
horizontal draft carburetors
Horizontal draft carburetors

Downdraft Carburetors

  • Similarly, the downdraft carburetors are the one which supplies the air from the above portion of mixing chamber. Among the 3 types, downdraft carburetors are used mostly because of some advantages.
  • The gravity assists the air-fuel mixture engine will pull better at lower speeds. Because of this, the engine with downdraft carburetors can achieve higher volumetric efficiency.
  • The only disadvantage it has the jet overflow because of the possibility of leakage directly into the inlet manifold if the float is defective.
downdraft carburetors
Downdraft carburetors

The use of carburetors was stopped due to introduction of the fuel injection technology. There are some advantages and disadvantages of the carburetor let’s know them now.

Advantages of Carburetors

The advantages of carburetors are as follows,

  • The carburetors parts are not that much of expensive as compared with fuel injectors.
  • The more air-fuel mixture can be acquired using carburetors.
  • If considered a road test, carburetors have more power and precision.
  • They are not restricted to the amount of gas pumped by the fuel tank. Cylinders can pull more fuel from carburetors achieving higher power.
  • Allows users to tune up carburetors according to requirements.
  • They can be serviced separately without touching the engine.

Disadvantages of Carburetors

There are disadvantages of carburetors as well, these are,

  • At low speeds, the carburetor is not capable to provide the proper mixture.
  • The changes in the atmospheric conditions affect the working efficiency of the carburetor.
  • More fuel consumption compared to fuel injectors.
  • Components like the diaphragm of carburetors are delicate and easily get damaged.
  • Engine smoothness is affected due to air-fuel mixture fluctuation.

There are various disadvantages compared to fuel injection. Carburetors are now becoming obsolete as even the small bikes are incorporating fuel injection technology because of advantages and advancements in fuel injection technology.

Manufacturers of Carburetors

There are a lot of carburetors manufacturers across the globe, few of them are listed below,

  • AMAL,
  • Carter,
  • Dell’Orto
  • Hitachi,
  • Holley,
  • Keihin,
  • Solex
  • SU Carburettors,
  • Weber carburetor,
  • Zenith,
  • Fretlog India
  • Nikki India Fuel Systems
  • Pacco Industries Corp.
  • Noble Industries. India.
  • Craftsman Automation Pvt., Ltd.
  • Apa Engineering


Hence, from this lesson, we have learned the basics of the carburetor, its working principle, types, and many more. If you have any doubt, please write to us.

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