Carburetor 101: Maintenance and Cleaning

DIY

Engines and Components

Jefferson Bryant

November 30, 2023

Jefferson Bryant

November 30, 2023

Modern vehicles use electronic fuel injection to feed fuel to the engine. This is an incredibly complex system of modules, wires, sensors, injectors, and computers that efficiently use air and fuel to maximize fuel economy and power. It can also be challenging to troubleshoot and repair without expensive scanning tools.

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All of this was developed to replace one component that did the same job: the carburetor. Almost every vehicle built before 1987 came with a carburetor, so a little maintenance will help it operate as well as it can.

To gain access to the carb, start by removing the air cleaner's wing nut. — The carburetor lives under the air filter like this one; remove the wing nut to access the carburetor itself.

What Does a Carburetor Do?

Unlike fuel injection, a carburetor is a mechanical metering device that uses several scientific principles to draw fuel into the engine.

This 1965 Mercury Park Lane's carburetor desperately needs to be cleaned. — On this 1965 Mercury Park Lane, the carburetor has seen much better days. It is incredibly dirty and gummed with residue.

An engine is nothing more than an air pump. The four-cycle action of the pistons and valves draws air in, squeezes it, combusts the air/fuel mixture, and then pushes the exhaust out. Without a carburetor, your engine can still run wide open and completely uncontrolled as long as you squirt fuel into the intake. The carburetor controls both air and fuel flow.

Take photos of the vacuum lines before removing them to clean the carburetor. — There are a lot of lines on a carburetor, and most of them are vacuum lines. If you have to remove the carburetor, take pics so you can refer to them later.

The carburetor controls airflow via the butterfly blade at the base. Air flows through the barrels (the open holes in the top of the carb), over the venturis (the small halo rings at the top of each barrel), and through the venturi effect, drawing fuel into the engine. A mechanical fuel pump in the carburetor adds an extra squirt of fuel when you step on the gas pedal. This provides the fuel necessary for the sudden burst of air that flows in as the blades open.

Potential Carburetor Problems

Carburetors are intricate mechanical devices with many moving parts that get sticky over time. Many carburetors, with basic maintenance and regular use, can last decades without being rebuilt.

Carburetor choke opened for inspection — Open the choke and look inside the carburetor for varnish and grime. Carburetor cleaner washes varnish away instantly.

Regular use is the key to carburetors. When fuel sits in the fuel bowls, it slowly evaporates, leaving behind a sticky varnish that doesn’t melt off with fresh fuel. Over time, this buildup causes the internal components to stick and bind, reducing the effectiveness of the carburetor.

Ethanol fuels have the added issue of corroding the metal. As ethanol fuels evaporate, they leave behind a white powdery substance that ruins everything. This is less of an issue if you run the engine at least weekly and long enough to run the old fuel out of the bowl. Adding an ethanol fuel stabilizer - opens in new window or tab. is highly recommended if you use ethanol fuels and your vehicle sits for long periods.

Carburetor Maintenance

You should check your carburetor every 15,000 to 20,000 miles or every six months, whichever comes first. External cleaning is quick and easy, allowing you to look for failing hoses, clamps, linkages, or other components.

Carburetor cleaning - linkage — The linkages should move freely and return to the closed position. If they don’t, they need cleaning. Make sure the throttle return spring is connected.

You should also run a bottle of fuel system cleaner - opens in new window or tab. through the gas tank simultaneously. This helps clean any internal varnish or buildup inside the carburetor and fuel system.

Cleaning a Carburetor

You only need a few simple tools to clean a carburetor:

A can of carburetor cleaner
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A small, stiff brush

If you’re doing a deep clean that requires removing the carburetor from the engine, add a carburetor mounting gasket - opens in new window or tab. to the list of things to buy before beginning.

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Light-duty cleaning can be done with the carburetor on the engine. Heavy cleaning may require removal. The following is regarding the most common downdraft style carburetor. Side-draft carburetors such as SU - opens in new window or tab. and Weber - opens in new window or tab. styles are different, and while cleaning and removal are similar, some differences are not reflected here.

carburetor cleaning - air/fuel mixture screw — This is an air/fuel mixture screw, don’t make any changes to any of the adjustments unless you know what you are doing. It is very easy to knock a carb out of adjustment, making your engine run poorly.

Disconnect the battery negative terminal.
Remove the air cleaner by removing the wing nut on top of the air cleaner. Some European cars use a different style of filter that may bolt or clamp on.
Examine the carb for signs of leaks, varnish, and sticky deposits.
Operate the throttle linkage while looking down the barrels. A little shot of fuel should come out of the venturi jets in the front barrels (if it’s a four-barrel carb). Make sure the lower butterflies open.
Look at the hoses and connections on the carburetor. Any cracked, split, loose, or leaking hose should be replaced. If there are any uncapped ports, locate the hose that belongs there or place a vacuum cap on it.
Spray the carburetor with the carburetor cleaner spray. Cover the entire carburetor, then wipe it down with a clean shop towel.
Scrub any sticky linkages and fittings with the stiff brush. Spray with carburetor cleaner, then wipe them off.
Spray a little carburetor cleaner into the barrels while operating the throttle by hand.
Replace the air filter and start the engine.

Time to rebuild the carburetor. — If you see signs like this crusty crud between the different layers of the carburetor body, the gaskets have failed, and it needs to be rebuilt.

If the engine is not running well, the jets are not squirting a strong stream of fuel, the carburetor is leaking around the seals, or the engine has been sitting with fuel in the bowls for a year or more (six months with ethanol fuel), then it likely needs to be rebuilt. While a rebuild is complicated enough to be detailed separately, we will cover the removal process here.

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Carburetor Removal

Sometimes, you must pull the carburetor to get it cleaned or rebuilt. This process varies by engine, but the basics are the same.

carburetor cleaning 8 - leaky gaskets — This dirty yellow residue is varnish from leaky gaskets on the carburetor body.

Disconnect the battery negative terminal.
Remove the air cleaner by unthreading the wing nut at the top.
Take a few pics of each side of the carburetor for reference. It is very easy to lose track of the individual connections and linkages.
Disconnect the throttle linkage and kickdown/TV cables. This may be a ball stud that pops off or a slip-on ring secured with a cotter pin.
Remove the fuel line. Most carburetors use a hose clamp on a flexible line, but it could be a hard line requiring a line wrench. Using an open-end wrench could strip the nut.
Disconnect all vacuum lines and label them as you do so they will be easier to reinstall later.
The carburetor should be free from all lines and linkages at this point. Unbolt it from the intake manifold. Some carburetors have studs, and some use bolts. Be careful with the nuts, bolts, and washers you remove, as you do not want anything falling into the intake while the carburetor is off.
Carefully lift the carburetor to remove it. It will likely be stuck to the gasket, so you may need a small flat-blade screwdriver or thin pry tool to free it. Set the carburetor aside once removed.
Remove the old intake carburetor gasket. Use a scraper to clean any remaining material off the mounting flange. Stuff a shop towel into the intake to prevent debris from falling into the engine.

At this point, the carburetor is ready for deep cleaning or rebuilding. Installation is the reverse of removal. If you rebuild your carburetor, you will need to tune it.

Use a line wrench to remove a fuel hard line before cleaning the carburetor. — When removing a fuel hard line, make sure you use a line wrench like this to avoid stripping the fitting nut.

If you take the time to regularly check and clean the external body of the carburetor and linkages, along with adding a fuel system cleaner to the fuel, you will keep internal problems at bay for years. Many 60-plus-year-old engines still have the original carburetor in place, having never been rebuilt.

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About the author

Jefferson Bryant

Jefferson Bryant, a lifelong gearhead, got his first car when he was two: a Fisher-Price convertible. An industry veteran for over 30 years, he started in car audio, working his way up to product designer for Rockford Fosgate. Bryant has written nine books, countless magazine articles, and produces tech videos on YouTube. A hardcore Buick man, his pride and joy is a 1971 Buick GS Convertible which he swapped with a Procharged Gen V LT1. He even brought his kids home from the hospital in it.

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This article is meant to provide general guidance only. Automotive maintenance, repair, upgrade, and installation may depend on vehicle-specifics such as make and model. Always consult your owner's manual, repair guide for specific information for your particular vehicle and consider a licensed auto-care professional's help as well, particularly for advance repairs.