What Is a Carburetor?

Key Points

• The fuel system in a vehicle includes a carburetor, which mixes gasoline and air to create combustion.
• Carburetors were commonly used in vehicles until the late 1980s when fuel injection systems became more popular.
• Carburetors work by creating a vacuum to draw in air and fuel, with valves and a throttle controlling the air-fuel mixture.
• Fuel injection systems replaced carburetors and use electronic processes to deliver the correct air-fuel mixture.
• Signs of a failing carburetor include reduced engine performance, backfiring/overheating, black exhaust and difficulty starting.

The fuel system in a vehicle features many parts that help store and supply the engine with the fuel it needs to run. Gasoline gets mixed with air, and this mixture burns inside cylinders to generate power that gets the car moving.

A carburetor is a part responsible for mixing these components in the right amounts so the combustion process can occur. While carburetors aren’t used in late-model vehicles, they were used for many years until the introduction of electronically controlled fuel-injection systems.

Keep reading to learn more about how carburetors work and how they were used to deliver fuel to the engines of many vehicles through the years.

How Does a Carburetor Work?

For a simple picture, a carburetor is a device that is fitted above an engine’s cylinders and has both an air and fuel pipe attached to it. A vacuum gets created, which is what the carburetor relies on to help draw air and fuel into the cylinders.

When air gets pushed down the pipe, it passes through a narrow opening known as a venturi. The air must speed up to pass through this area, which causes a drop in pressure. It’s this pressure that allows air to be drawn in through the fuel pipe.

Below and above the venturi are two valves that are important for adjusting the air-fuel mixture. The top of the carburetor features the choke, which regulates how much air is coming into the pipe. The second valve below the venturi is called the throttle, which can open and close to allow different amounts of air to enter the engine.

A throttle that’s more open will allow an increased amount of air to flow through the carburetor, which will bring in more fuel. This allows an engine to release more energy and have more power.

Inside the carburetor there’s a part known as the jet, which is an opening that allows fuel from the float chamber to mix with air before it enters an engine’s cylinders. Float chambers hold a small amount of fuel in them and allow it to flow to the jet as needed.

Types of Carburetors

Various types of carburetors have been developed over the years and were designed to meet specific performance requirements. The primary types of carburetors include fixed-venturi and variable-venturi.

Fixed-venturi carburetors

Fixed-venturi carburetors are the simplest and most common type, characterized by a fixed-size venturi, a constriction in the airflow path that creates a pressure drop, drawing fuel into the air stream. This type of carburetor maintains a consistent air-to-fuel ratio, making it suitable for a wide range of engines.

The simplicity and reliability of fixed-venturi carburetors make them popular in many small engines, such as those found in lawnmowers and motorcycles.

Variable-venturi carburetors

Variable-venturi carburetors, also known as constant-depression carburetors, offer more precise control over the air-to-fuel mixture by adjusting the size of the venturi based on engine demand. These carburetors typically use a piston or diaphragm that moves in response to engine vacuum, altering the venturi size and consequently the fuel flow. This design allows for better fuel economy and performance, especially at different engine speeds and loads.

Variable-venturi carburetors are often found where the demands for power and efficiency are more complex.

Benefits of Carburetors

While most modern vehicles use fuel injection systems, carburetors offered several benefits that made them a crucial component in internal combustion engines. These include:

• Simplicity: Compared with electronic fuel injection systems, carburetors have a basic design that is easy to understand and manufacture.
• Cost-effective: Due to their simplicity, carburetors have lower production costs than fuel injection systems.
• Easy to maintain: With their straightforward design, carburetors are relatively easy to maintain and repair and most issues can be addressed with basic mechanical skills without the need for specific diagnostic equipment.

Cons of Carburetors

There are disadvantages to using carburetors, which have led to their replacement by more modern fuel injection systems (which you’ll learn about in a minute). Drawbacks to using carburetors include:

• Inefficiency in fuel delivery: Carburetors can have issues delivering the right amount of the air-fuel mixture needed for optimal engine performance, unlike fuel injection systems that can adjust fuel delivery in real-time based on sensory inputs.
• Susceptible to environment: Environmental factors such as temperature and altitude changes can negatively impact carburetor performance and, in turn, affect engine performance and efficiency.
• Frequent maintenance: While carburetors may be easy to maintain, they do require regular maintenance to function correctly, which can be time-consuming. Components can become clogged or simply wear out relatively quickly compared to fuel injection systems.
• Emissions control challenges: Compared to fuel injection systems, carburetors are less effective when it comes to controlling exhaust emissions.

Carburetor vs. Fuel Injection

Carburetors were invented in 1888 by Karl Benz, one of the founders of Mercedes-Benz. It was a groundbreaking development for the industry at the time, and the technology was used by car manufacturers up until the late 1980s, when carburetors began getting replaced by fuel injection systems.

The main difference between carburetors and fuel injection systems is that fuel injection systems introduce an electronic process for delivering the necessary mixture for combustion.

While modern injection systems began being widely used in the 1980s, NASCAR vehicles still featured carburetors until 2012.

Fuel injection systems feature a range of parts that include a computer, fuel injectors, oxygen sensors and pressure regulators that work to ensure the right blend of air and fuel is delivered to the combustion chamber.

Electronic fuel injection allows for precise control over the amount of fuel being pushed into the cylinders. Fuel exits the fuel tank, traveling through lines toward the engine.

There is also the electronic control unit (ECU), which determines the exact amount of fuel the engine needs based on the volume of air entering the engine and the optimal air/fuel ratio determined by the specific manufacturer.

Carburetor vs. Throttle Body

There is sometimes confusion surrounding the differences between a throttle body and a carburetor. Both are components that manage the air and fuel entering the engine, but they operate differently and with different technologies.

Carburetors have the primary function of mixing fuel and air in the correct ratio before they enter an engine’s combustion chambers. The throttle body, on the other hand, helps to control the amount of air entering the engine and doesn’t mix fuel with air – this is done separately by fuel injectors within a modern fuel injection system.

Carburetors use mechanical means for the process, while the throttle body works in conjunction with an electronic fuel injection system. Carburetors are still found in small engines, motorcycles and classic cars, whereas you can find a throttle body in more modern automotive engines or for industrial and high-performance applications.

Signs of a Failing Carburetor

Although newer vehicles aren’t being produced with carburetors, there are older models on the road that still have them.

If you happen to be driving a car that has a carburetor, it’s important to know some of the signs that it might be failing. Over time, wear and tear can occur to the carburetor, and it might need to be serviced or replaced. Some symptoms include:

• Reduced engine performance: The most common sign something’s wrong with the carburetor would be reduced engine performance. The carburetor wearing down can cause the air-fuel mixture to be off, resulting in a decrease in acceleration and power.
• Backfiring/overheating: These are also common symptoms of carburetor failure. A lack of fuel in the air-fuel mixture can cause these two issues to happen, which in severe cases can cause severe engine damage.
• Black exhaust: There should never be black smoke coming from the exhaust! This is likely a sign that the engine is using too much fuel, also known as fuel-rich. This can come from a carburetor that is worn and delivering excess amounts of fuel.
• Difficulty starting: A common problem stemming from a worn carburetor is hard starts or difficulty starting the engine. Issues messing up the air/fuel ratio can result in these problems.

If you experience any of these issues, it’s a good idea to take your vehicle to a trained technician who can diagnose and fix your problem. You don’t want to cause more damage to your engine by ignoring possible signs of carburetor failure!

Carburetor FAQs

What are the different types of carburetors?

The two main types of carburetors include the fixed-venturi and variable-venturi systems. In fixed-venturi systems, the velocity of the airflow is used to regulate the flow of the fuel, whereas in variable-venturi systems, the flow of raw fuel is controlled mechanically, and airflow is regulated with fuel flow.

How does a carburetor differ from a throttle body?

Carburetors mix fuel and air in the correct ratio before this mixture enters the engine’s combustion chambers. In contrast, the throttle body regulates the amount of air entering the engine and does not mix fuel with air.

How does a carburetor affect the performance of a vehicle?

A carburetor significantly impacts the performance of a vehicle by influencing several key aspects of engine operation, including fuel delivery, throttle response, engine efficiency and emissions.

Learn More About Fuel Injection at UTI

Want the opportunity to train in hands-on labs with fuel injection systems and other car equipment?¹⁸ The Automotive Technology program at Universal Technical Institute (UTI) teaches the fundamentals of maintenance and repair on a variety of vehicle types through courses that combine interactive labs with online instruction.

You can graduate in less than a year with the skills you need for an in-demand career in the industry.^1,7 As a bonus, you could gain the knowledge to make repairs on your own vehicle. Request more information today!

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