Car Carburetor Guide

carburetor or carburettor is a device that mixes air and fuel for internal combustion engines in the proper ratio for the purpose of combustion. It is sometimes colloquially shortened to carb in the UK and North America or carby in country of Australia. To carburate or carburet (and thus carburation or carburetion, respectively) means to mix the air and fuel or to equip (an engine) with a carburetor for that reason.

Carburetors have largely been supplanted in the automotive and, to a lesser extent, aviation industries through fuel injection. They are still popular on small engines for lawn mowers, rototillers and other equipment.

Basic

Cross-sectional schematic of a downdraft carburetor

A carburetor usually consists of an open pipe through which the air passes into the inlet manifold of the engine. The pipe is in the form of a venturi which narrows in section and then widens again, causing the airflow to increase in speed in the narrowest part. Below the venturi is a butterfly valve known as the throttle valve — a rotating disc that can be turned end-on to the airflow, so as to hardly restrict the flow at all, or can be rotated to (almost) completely to block the flow of air. This valve controls the flow of air by the carburetor throat and thus the quantity of air/fuel mixture the system will deliver, thereby regulating engine power and speed. The throttle is mostly connected through a cable or a mechanical linkage of rods and joints or rarely by pneumatic link, to the accelerator pedal on a car, a throttle lever in an aircraft or the equivalent control on other vehicles or equipment.

Fuel is introduced into the air stream with small holes at the narrowest part of the venturi and at other places where pressure will be lowered when not running at full throttle. Fuel flow is adjusted through means of precisely calibrated orifices, referred to as jets, in the fuel path.

What does a carburetor do? The carburetor has different functions: 1) it combines gasoline as well as air to create a highly combustible mixture, 2) it controls the ratio of air and fuel, and 3) it controls the speed of engine. How a carburetor mixes fuel and air When the piston moves down the cylinder on the intake stroke it takes air from the cylinder and intake manifold. A vacuum is created that takes air from the carburetor. The airflow across the carburetor causes fuel to be drawn from the carburetor through the intake manifold past the intake valves and into the cylinder. The amount of fuel mixed into the air for obtaining the required air to fuel ratio is controlled by the venturi or choke. When air flows across the venturi its speed increases and the pressure drops. This results in the fuel to be sucked into the air stream from a hole or jet. When the engine is at idle or is at rapid acceleration, there is not enough air passing through the venturi to draw fuel. To overcome these problems other systems have to be used. Delivering gasoline to the carburetor Gasoline is delivered to the carburetor with the help of the fuel pump and is stored in the fuel bowl. For keeping this level of fuel stored in the bowl constant under all conditions, a float system is used. A float operated needle valve and seat at the fuel inlet is used for controlling the fuel level in the bowl. If the fuel level drops beyond a certain level the float lowers and opens the valve letting more fuel in. Whenever the float rises, it pushes the inlet valve against the seat and shuts off the flow of fuel into the bowl.   Controlling the speed of the engine The throttle controls the speed of the engine through controlling the amount of air fuel allowed in the engine. The throttle is a butterfly valve which is located after the venturi and is opened by pressing on the gas pedal. The farther the valve is opened the more air/fuel mixture is let into the engine because of which the engine runs faster. At low engine speeds when the throttle is little bit open, there is not enough air flow to be pulled into fuel. Ports Two ports are used for solving this problem. One port found in the low pressure area and the idle port located below. At low engine speeds, both ports take fuel to keep the engine running. As engine speed rises, fuel from the 2 ports decreases until it stops completely. Handling low speeds When the engine is idle, there is a low amount of air flowing through the venturi because the throttle valve is closed. The idle port allows the engine for operating under this condition. Fuel is forced by the idle port because of a pressure differential between air in the fuel bowl and vacuum below the throttle valve. Idle fuel mixture is regulated by an adjustable needle valve. Handling high speeds At higher engine speeds ,more fuel is drawn out from the main nozzle. Fuel comes from the fuel bowl by the fuel nozzle and into the throat of the carburetor where it mixes with air.
Types of carburetors There are 3 basic types of carburettors which are in use today. They are -one barrel, two barrel, and four barrel. Usually, the type of engine and its use will dictate which carburetor is used. In high performance engines, multiple carburetors might be used to deliver the amount of fuel required. No matter what type of carburetor your engine is using, National Carburetors is your source for high quality carburetors.

How to Replace a Carburetor on Most Cars

A carburetor replacement consists of the air cleaner, vacuum hose, car fuel line, and a number of other parts. This complex procedure varies according to vehicle.

A carburetor’s function is used to meter incoming air, mix the appropriate amount of fuel with that air, and deliver the mixture to the engine. When that air-to-fuel ratio is off, this results in several different running conditions. There might be a loss of power, the engine may run too hot, and fuel economy can suffer. These are just some of the effects of a poorly functioning carburetor.

The process of replacing a carburetor that is given below is meant as an overview, not a tutorial on any one model in particular. There are multiple types of carburetors, and all manufacturers are not using the same types. Some manufacturers might even use different brands of carburetors on the same model and year of a given car. This is why this overview has been written in a way that should cover all of the basics of carburetors in general.

  • Warning: While servicing any part of the fuel system on a vehicle, it is very important to take precautions to avoid fires. Fuel is extremely flammable and needs preparation to avoid any injuries. It is highly recommended that a fire extinguisher to be close to you at all times. It is also recommended that you wear safety glasses as fuel is caustic and can cause damage to the eyes very easily. Gasoline is also hazardous for inhaling. It is recommended that repairs needs to be performed in a well-ventilated area.

Part 1 of 10: Disconnecting the battery cables

Materials Required

  • Camera
  • Extension set
  • Flare nut wrenches
  • Light solvent, such as brake clean or alcohol
  • Marker
  • Masking tape
  • Pliers
  • Scraper or razor blade
  • Screwdrivers
  • Shop rags or towels
  • Small fuel-safe bowl or tray
  • Small pry bar
  • Socket set
  • Wrench set

While working on a fuel system, it is always a good idea to disconnect the battery cables to avoid any sparks that may be caused by accidentally touching electrical connections with tools or other surfaces. And as the process of changing a carburetor will involve contact with gasoline in both its liquid and gaseous forms, limiting the chance for a spark is highly important.

Step 1: Locate the battery and remove the cables. Find the battery under the hood of the vehicle and detach both the positive (+) as well as negative (-) battery cables.

Step 2: Secure the cables. You will have to shield and secure the cables in a way that they cannot inadvertently make contact with the posts on the battery while you are making repairs. Cover the terminals of the cable and secure them out of the way.

Part 2 of 10: Removing the air cleaner and any air intake ductwork

Most carburetors consist of an air cleaner assembly that covers the air intake side of the carburetor. This air cleaner assembly usually houses an air filter to keep dirt and debris out of the engine. This filter is mostly a folded paper element or a sponge type of material. The cover of the air cleaner assembly is generally held in place by screws, bolts, clips, or any combination of these.

Additionally, there might be ductwork, or hoses, that run from the air cleaner assembly to a source of fresh air on the front of the vehicle.

Step 1: Identify the air cleaner assembly and all related ductwork. Visually identify all of the parts which make up the air cleaner assembly and the ductwork that will need to be removed in order to access the carburetor.

Step 2: Remove the air cleaner assembly and all ductwork. Remove all the parts associated with the air cleaner assembly and its ductwork using tools according to the type of fastener holding the parts in place. Set these components off to the side and out of the way.

Part 3 of 10: Identifying and removing all of the vacuum hoses and other connections

A carburetor functions through mechanically reading signals from the engine to determine how much load the engine is under. One of the ways this load is determined is through getting a signal from the intake manifold in the form of a vacuum inside the intake manifold. This signal is delivered to the carburetor through the vacuum hoses.

Depending on the style of carburetor used in the vehicle, there will be one or more vacuum hoses connecting to the carburetor.

Step 1: Visually identify all of the hoses attached to carburetor. Look around the perimeter of the carburetor and help to identify all the hoses that connect to it.

  • Tip: Taking pictures of the hoses in their original configuration with a camera or phone is a good idea as you may then refer back to the pictures upon reassembly.
  • Tip: If required, hoses and any subsequent connections can be labelled with masking tape and a marker to help you remember the place and function of each one.

Step 2: Remove all of the hoses. Remove all of the hoses that are attached to the carburetor and secure them off to the side of the work area.

Part 4 of 10: Removing any linkages

Find the throttle linkage that connects the throttle pedal inside the car to the carburetor, which will be attached to the side of the carburetor. This linkage helps to control the butterfly valve on the carburetor. The butterfly valve then controls the amount of air entering the carburetor, thus, signalling how much fuel will be needed by the engine. The throttle linkage can be held on by any combination of clips, bolts, or screws.

Step 1: Visually identify the linkage and its bracketry. Look on all sides of the carburetor and find the throttle linkage and any of its bracketry that will need to be removed in order to separate the carburetor from the intake manifold.

Step 2: Remove the linkage and bracketry. Disconnect the throttle linkage and any bracketry that might mount it to the side of the carburetor. Remove the linkage as well as the bracketry and secure it off to the side.

Part 5 of 10: Disconnecting the fuel supply line

The carburetor utilizes fuel from the fuel tank to supply gasoline to the engine. There is usually a hose or metal tube for this, called the fuel supply line that connects to the carburetor.

This line might be held onto the carburetor by a hose clamp (for a rubber hose) or it may be threaded directly into the carburetor (in the case of a metal line).

Step 1: Release any residual fuel pressure in the fuel line. Gently break the line free and let off any kind of residual pressure that might still be present.

Depending on the type of fuel system being employed, there might still be pressurized fuel in the fuel line. When removing the fuel line, you need to have something available to catch the escaping fuel when the line is removed.

If there is room, a bowl or tray placed underneath the line would be best option. If room is limited, a shop rag or towel might be placed below the line in an effort to catch as much fuel as possible.

Step 2: Remove the fuel supply line. Remove the fuel supply line through removing any necessary clamps, bolts, nuts, or other fasteners necessary to separate the fuel supply line from the carburetor.

Step 3: Drain the fuel supply line. Drain as much residual fuel out of the line as possible for avoiding leakage.

Step 4: Move the fuel supply line off to the side. Position the line off to the side in such a way that will not allow fuel to drip on the work area.

Part 6 of 10: Removing the old carburetor

The carburetor is held onto the intake manifold through any combination of bolts, nuts, studs, and screws. These fasteners can be found anywhere on the carburetor, such as around the base or even through the center of the carburetor housing.

Step 1: Identify and remove the carburetor mounting hardware. Find all hardware that secures the carburetor to the intake manifold.

Use the appropriate tools for removing the mounting hardware.

Step 2: Separate the carburetor from the intake manifold. Lift the carburetor to make it free from the intake manifold.

If there is resistance to lifting the carburetor off, a small pry bar or screwdriver can be used for gently prying the carburetor free.

  • Warning: Be very careful while prying on the carburetor as the body and flanges are fragile and can be damaged or broken if you do not use caution. Make sure to never force anything into the seam between the carburetor and the intake manifold. This can result in damage to the flanges on either the carburetor or the intake manifold, which in turn can cause vacuum leaks and lead to rough running conditions later on.

Step 3: Block off the intake manifold opening. Put shop rags or towels in the intake manifold opening to keep debris from falling into the engine.

Take caution of not putting them so far that they cannot be retrieved when it is time to install the replacement carburetor.

Step 4: Remove the old gasket, if applicable. Remove the gasket which is between the carburetor and intake manifold.

  • Note: Not all manufacturers are using a gasket. If no gasket is present, contact a certified mechanic. In fact, throughout this process, you can always Ask a Mechanic for getting a quick, detailed advice from one of our certified technicians.
  • Tip: If the gasket sticks to the surface of the carburetor or the intake manifold, a scraper or razor blade can be used to gently remove the gasket. Ensure not to gouge the surfaces as this can lead to a vacuum leak later on.

Part 7 of 10: Prepare to install the replacement carburetor

Step 1: Clean the carburetor and intake manifold mounting flanges. With a light solvent, clean the flanges of both the intake manifold and the replacement carburetor.

Remove all dirt and debris to ensure a leak free seal.

Step 2: Install a new gasket, if applicable. Insert a new carburetor gasket on the intake manifold (most replacement carburetors come with a new gasket).

Place the new gasket onto the intake manifold, oriented so it sits in between the intake manifold and the carburetor in the same way as the old gasket with the cut-outs in the new gasket and matches up with the openings in both the intake manifold and the carburetor.

Step 3: Compare the old carburetor to the new carburetor. Take a few minutes for comparing the old carburetor to the replacement carburetor, making sure that they have the same features, linkages, mounting points, fuel inlet style, and are at least in the same general position.

Step 4: Transfer parts from the original carburetor into the new carburetor. Transfer any part from the old carburetor to the replaced one.

This is usually easier to do before the replacement is mounted on the intake manifold.

Step 5: Remove any shop rags or towels. Remove any shop rags or towels that might have been previously placed into the intake manifold.

Part 8 of 10: Installing the replacement carburetor

Step 1: Set the new carburetor onto the intake manifold. Put the replacement carburetor back onto the intake manifold.

Step 2: Complete the installation of the carburetor. From here, follow the removal process in a reverse order to the point where all vacuum hoses, the fuel line, and the throttle linkage have been reinstalled.

Part 9 of 10: Checking the carburetor for any leaks

Once the carburetor has been inserted and all of the fuel supply lines, vacuum hoses, and linkages have been reinstalled, you can start to check the system for any leaks.

Step 1: Reconnect the battery. Reconnect both the positive (+) as well as negative (-) battery cables.

Step 2: Build pressure to the carburetor. Depending on the type of fuel system you have (electric versus mechanical), just apply fuel pressure to the system.

  • Note: Vehicles with an electric fuel pump will usually only require the ignition key be placed into the run position to inspect for a leak. Vehicles with a mechanical fuel pump might need the engine to be cranked before fuel pressure can be achieved. The engine does not necessarily need to run, just cranked long enough for building up pressure in the line to check for leaks.

Step 3: Inspect the carburetor for leaks. Check the carburetor, hoses, lines, and general area around the carburetor for any signs of fuel leakage.

  • Tip: Listen for a whistling sound around the carburetor, as this might also signal that there is a vacuum leak.
  • Warning: If a leak is detected, just immediately shut the engine off and be careful not to cause any sparks. Clean up the spilled fuel as soon as possible.

Step 4: Repair any leaks found. Tighten any fittings or fasteners as important to seal any kind of leaks.

Step 5: Start the engine again. Once any of the leaks are taken care of, start the engine and check the functionality of the new carburetor.

Part 10 of 10: Completing the installation

Step 1: Reinstall the air cleaner and any associated ductwork. Replace the air cleaner and any ductwork which was removed during disassembly.

Step 2: Give a final visual inspection. Take a couple of minutes for visually inspect the repair area ensure there are no tools still lying in the engine compartment.

Ensure that all fasteners are tight and that nothing has been left loose.

Step 3: Test drive the car. Road test the vehicle for verifying that it is functioning correctly.

  • Warning: If at any time during the road test, a fuel smell is detected, just pull off of the side of the roadway in a safe location and shut the engine off immediately. Inspect the vehicle for any leaks that might have arisen and repair as necessary.

Video of How to Install the Carburetor

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