How does a carburetor work?

Would you know a carburetor if you saw one? Iuliia Burmistrova / Getty Images

In the simplest terms, car engines work by converting the energy stored in gasoline to mechanical force. This is accomplished via combustion (hence the "internal combustion engine" name), which requires the mixing of gasoline with air.

Before the rise of fuel injection systems, almost all car engines performed this task with a device called a carburetor, but given the relative scarcity of the older technology, you might be prompted to first ask: What is a carburetor?

Contents

What Does a Carburetor Do in an Engine?
Inside a Carburetor
Carburetor Tuning
What Is Fuel Injection?

What Does a Carburetor Do in an Engine?

A carburetor's task is to mix the right amount of gasoline and air together. Despite what you might have assumed, gasoline, when in its liquid form, isn't actually flammable. In fact, it's the fumes that ignite.

A carburetor takes the liquid gasoline from the gas tank and mixes it with air, which then travels to the combustion chamber, where the mixture is ignited by the spark plug.

Of course, the air fuel mixture has to be just right. A well-performing engine needs the Goldilocks-like calibration of the carburetor. If there is not enough fuel mixed with the air, the engine "runs lean" and either will not ignite, or potentially damage the engine.

If there is too much fuel mixed with the air, the engine "runs rich" and either will not run (it floods), runs very smoky, runs poorly (bogs down, stalls easily), or at the very least wastes fuel.

Inside a Carburetor

Even in the era of electronic fuel injection, many small-engine machines still use carburetors (aka carbs). Since the carburetor on a chain saw is simpler than most carbs, it's a good place to demonstrate the basics. It really has only three situations that it has to cover:

It has to work when you are trying to start the engine cold.
It has to work when the engine is idling.
It has to work when the engine is wide open.

No one operating a chain saw is really interested in any gradations between idle and full throttle, so incremental performance between these two extremes is not very important. In a car, the many gradations are important; this is why a car's carb is a lot more complex.

Here are the parts of a carburetor:

A carburetor is essentially a tube.
There is an adjustable plate across the tube called the throttle plate that controls how much air can flow through the tube. You can see this circular brass plate in photo 1.
At some point in the tube there is a narrowing, called the venturi, and in this narrowing a vacuum is created. The venturi is visible in photo 2
In this narrowing there is a hole, called a jet, that lets the vacuum draw in fuel. You can see the jet on the left side of the venturi in photo 2.

Carburetor Tuning

The carb is operating "normally" at full throttle. In this case the throttle plate is parallel to the length of the tube, allowing maximum air to flow through the carb. The air flow creates a vacuum in the venturi, and this vacuum draws in a metered amount of fuel through the jet.

Hi and Lo Screws

You can see a pair of screws on the right top of the carb in photo 1. One of these screws (labeled "Hi" in the case of the chain saw) controls how much fuel flows into the venturi at full throttle.

When the engine is idling, the throttle plate is nearly closed (the position of the throttle plate in the photos is the idle position). There is not really enough air flowing through the venturi to create a vacuum.

However, on the back side of the throttle plate there is a lot of vacuum (because the throttle plate is restricting the airflow).

Idle Jet

If a tiny hole is drilled into the side of the carb's tube just behind the throttle plate, more fuel can be drawn into the tube by the throttle vacuum. This tiny hole is called the idle jet. The other screw of the pair seen in photo 1 is labeled "Lo" and it controls the amount of fuel that flows through the idle jet.

Both the Hi and Lo screws are simply needle valves. By turning them you allow more or less fuel to flow past the needle. When you adjust them you are directly controlling how much fuel flows through the idle jet and the main jet.

Choke Plate

When the engine is cold and you try to start it with the pull cord, the engine is running at an extremely low RPM. It is also cold, so it needs a very rich mixture to start. This is where the choke plate comes in.

When activated, the choke plate completely covers the venturi. If the throttle is wide open and the venturi is covered, the engine's vacuum draws a lot of fuel through the main jet and the idle jet (since the end of the carb's tube is completely covered, all of the engine's vacuum goes into pulling fuel through the jets).

Usually this very rich mixture will allow the engine to fire once or twice, or to run very slowly. If you then open the choke plate the engine will start running normally.

What Is Fuel Injection?

Almost all new cars sold today use electronic fuel injection systems. This is because a fuel injected engine provides better fuel economy and lower emissions. The changeover really started in the 1980s, when computer chips became compact and cheap enough to widely adopt in consumer vehicles.

A fuel injected car engine uses a computer to calculate the precise amount of air and fuel required for the engine. It uses sensors to determine what the right mixture would be, so all the variables can be taken into account: the temperature of the engine, the richness of the current mixture, the oxygen level in the intake air, and so forth.

Carburetor FAQs

What is a carburetor?

ÂÂTÂhe goal of a carburetor is to ensure that the fuel air mixture is appropriate. This allows the engine to perform optimally.

Do any cars still use carburetors?

Nearly all cars sold before 1990, as well as small equipment like lawn mowers and chain saws, use carburetors because they are simple and inexpensive. But fuel injection systems are becoming nearly universal because it provides better fuel efficiency and lower emissions.

How do you adjust Hi and Lo screws on a carburetor?

What happens when a carburetor goes bad?

If there is not enough fuel mixed with the air, the engine "runs lean" and either will not run or potentially damages the engine. If there is too much fuel mixed with the air, the engine "runs rich" and will either not run, run very smoky, run poorly or waste fuel.