Understanding the inner workings of a Carburetor

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What is a Carburetor?

A carburetor is a device that regulates the fuel and air ratio in a combustion engine. It is found in most older vehicles and small equipment like lawn mowers and rototillers.

It consists of a venturi that narrows and widens to push more air into the cylinders than normal. This is controlled by the throttle valve, which can be set to hardly restrict the flow or completely block it.


A carburetor is a device used to supply fuel and air to an engine. Its function is based on the theory of Bernoullii’s principle which states that the pressure of fluids will decrease when it flows through a constricted section.

In a carburetor, air is forced through a venturi throat that is cast in the carburetor bore. This restriction causes the air to speed up and creates a vacuum, which is strongest in the throat. This vacuum, however, exists only for a short distance before it disappears.

The air then passes through the nozzle, which is connected to a needle valve that regulates the amount of fuel coming into the system. When the level of fuel in the float chamber rises, the float will move closer to the open end of the needle valve and increase the flow of fuel through the system.

During operation, the needle valve will not be completely closed until it is moved backwards to idle cut off (ICO) or forward to rich mixture control. This enables the engine to be run under different loads and speeds while retaining a consistent air-fuel mixture.

There are several types of carburetors. Some are fixed while others have adjustable venturis. The main advantage of adjustable venturis is that they allow the engine to be tuned to run at different loads and speeds.

Another type of carburetor is a variable jet carburetor. These carburetors can be adjusted to adjust the size of their fuel jets depending on the engine’s requirements. The difference between the sizes of the jets is determined by the intake airspeed and the throttle position.

This allows the fuel to be mixed in a more consistent manner and reduces the chance of ice formation in the metering jet and carburetor. The ice can affect the manifold pressure and cause a drop in rpm, as well as an overall loss of power output.

Some carburetors also incorporate anti-icing technology. This involves ducting hot air into the venturi to prevent the build-up of ice and to keep the carburetor operating. This can be done pre-emptively as a prevention measure, but is most effective when applied to ice that has already formed.


The throttle of a carburetor is the control lever which controls the amount of fuel and air flowing in the carburetor. When the valve is opened, more fuel and air flows in, which makes the engine more powerful and accelerates the car faster.

There are different types of carburetors that use a variety of methods to control the supply of air-fuel mixture. They vary from those that use only a venturi to those that use jets and a choke.

In general, carburetors have two fuel wells that contain floats which act to maintain a pre-set level of fuel within the carburetor. As fuel is drawn out of the fuel wells, the floats drop and open the needle valves. The floats are also used to prevent the needle valves from being closed by fuel leakage.

Another common feature of a carburetor is the intake manifold vacuum or pressure that is generated by the throttle plate during idle. The vacuum created by the pressure of the throttle plate is applied to a small hole, the idle jet, drilled in the side of the carburetor tube just behind the throttle plate.

As a result, the air that passes through this tiny hole is drawn into the main jet in the middle of the carburetor body and emulsified or mixed with atmospheric air. This emulsion, along with the air correction jet in the main circuit, provides a good initial mix of air and fuel.

Some of the more sophisticated carburetors have additional air bleeds and progression holes that provide further emulsification for the fuel delivered via the primary jet, and a little more. These air bleeds and holes are designed to enhance the delivery of vaporized fuel and help prevent starvation during inverted flight.

Other carburetors utilize a variable metering closure in the intake air stream that is controlled by the pressure of the throttle plate, or intake manifold vacuum. This variable closure can be as close as a few millimeters to the front of the carburetor or as far back as the center. This type of metering closure allows the air-fuel ratio to vary over a broad range without affecting the engine’s output.


When your engine starts up, it uses air and fuel to power the combustion process. The carburetor is responsible for mixing the two ingredients together in the proper ratio.

The float is an essential part of the carburetor. It helps keep the level of fuel in the float chamber at a constant level. The float also works with a needle valve and seat to control the amount of fuel entering the chamber.

Unlike the needle valve, which is mechanical, the float operates on the Venturi effect. This means that pressure decreases as the float moves down into the venturi. When the float lowers, it opens the needle valve, allowing more fuel to enter the chamber. The float then rises to close the opening between the needle valve and the seat.

To check if the float is working properly, hold the carburetor up so that the float tang is resting on the spring loaded plunger in the float valve. Make sure it does not compress.

Look at the tip of the float needle to see if it has worn away from a cone shape, which indicates that the float needle is no longer functioning correctly. If the float needle is wearing, it should be replaced with a new one.

Another way to check if the float is working correctly is to measure the fuel level in the chamber against the float height. This can be done using an adapter inserted into the drain hole of the float chamber and fitted with a clear fuel line with graduations.

This method is easy to use and is a great way to ensure that the float and needle valve are operating correctly. It is also a great tool for diagnosing problems with the float needle valve and seat assembly or the float pin holes.

If the float is working correctly, it should raise and lower with the fuel level in the chamber. It should then move the needle valve and seat into their proper position to control the flow of fuel. The needle valve should have no kinks or other wear that will cause it to close off the fuel supply when not in use.


A carburetor is the specialized part of an internal combustion engine that allows fuel to be pulled through it and into the cylinders where it mixes with air and ignites. It is critical that this mixture is regulated properly so that the engine runs at peak performance.

The carburetor has several jets that are used to control the mix of air and fuel to a precise level. The most common carburetors feature an idle jet, main jet, and air jet. These jets operate from small throttle openings to a large number of rpms.

There are also many emulsion tubes that can be used to change the jet sizes for rich or lean conditions. Some emulsion tubes are adjustable while others can be set to the same jet size but only in one area, such as the pilot or main jets.

You can find jet sizing tools at most auto parts stores and some even have special scales. Using these tools, you can measure the diameter of the jet and determine its size.

Generally speaking, the larger the jet, the more fuel it can pull through the carburetor before hesitating. This can help your engine run smoothly when you are accelerating and improve fuel efficiency.

If your engine stalls when you accelerate or hesitates, it may be because of a jet that is too small. To fix this, you must adjust the main jet and the air correction jet (the jet that controls a certain ratio of air to fuel at higher rpms).

A good rule of thumb for the 34PICT/3 is an X130 or X132.5 for the main jet, and an 80-100Z for the air correction jet. If your engine is running very lean, you can try increasing the main jet to a higher number (X140), but if that doesn’t help, it’s probably time for a new carburetor.

A properly jetted carburetor can make a huge difference in the performance of your car or truck. It can increase horsepower, decrease fuel consumption, and allow you to drive it at a higher RPM without worrying about bogging down or stalling. Getting it right can be a long and difficult task, but if you put in the work, it will be worth it!

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