How to Adjust a Carburetor

Selecting the proper air-fuel mixture will help prolong the life of your car's engine. If your engine is running roughly, adjusting this mixture and choosing the right idle setting will ease engine pressure, preventing it from running too fast or too slow. Adjusting the carburetor in cars involves just a few simple steps that don’t require any complicated tools. See Step 1 for more detailed information.

Locate and remove the air filter. In most cars, you will need to remove the air filter to access the carburetor for adjustment. Open the hood and ensure the engine is turned off before removing the air filter and casing. Unscrew the fasteners and connectors, then remove the entire air filter assembly.

• The location of the air filter can vary based on the manufacturer, vehicle model, and engine type. Refer to your vehicle’s manual or shop guide for details.
• On most vehicles with a carburetor, the air filter casing is directly attached to the carburetor.

Locate the adjustment screws on the front of the carburetor. There will be two screws on the front of the carburetor used to adjust the air-fuel mixture.

• Typically, these screws have a flat head and can be adjusted with a screwdriver to regulate the air-fuel mixture within the carburetor.
• Some carburetors, like the Quadrajet used in most GM vehicles, have a special screw that requires a unique tool for adjustment. The Quadrajet carburetor uses a tool with a Double D-shaped head for adjustment.
• Other carburetors may feature four-cornered adjustment screws.

Start the engine and warm it up to normal operating temperature. Check the temperature gauge to ensure the engine has reached the appropriate operating temperature, while listening to the engine's sound to determine what adjustments are needed.

• Fuel-starved engine will produce a pinging sound at high RPMs when the throttle valve opens, which may indicate the engine is flooded with oil. Add the necessary fuel to the mixture.
• Over-fueled engine might not change the engine's sound but will emit a distinct smell. Reduce the fuel level accordingly.

Adjust the screws evenly and select the proper mixture. Adjusting the carburetor is similar to tuning a guitar or any stringed instrument. You want to turn the screws evenly and slowly until you find the most balanced setting. Regardless of whether the engine is too lean or too rich in fuel, begin by turning both screws a quarter turn counterclockwise at a time to lean out the mixture, then slowly turn them until you achieve a smooth and balanced air-fuel mixture.

• Adjusting the air-fuel mixture is a subtle art that requires careful attention and listening to your engine's sound. Slowly turn both screws while listening for smooth operation. Any pinging or knocking noise indicates a lean mixture. Continue turning until you find the ideal setting.

Reinstall the air filter. Once the carburetor adjustment is complete, place the air filter back in its original position, and your vehicle should be ready to go.

• If you need to adjust the idle speed, don’t replace the air filter until those adjustments are finished.

Locate the idle adjustment cable and the screw associated with it. This cable will connect from the throttle valve or accelerator pedal through the fan casing to the carburetor. Typically, if you cannot find the screw, refer to the user manual or the shop guide for details specific to your vehicle’s manufacturer and model year.

Start the engine and allow it to reach normal operating temperature. After adjusting the air-fuel mixture, let the engine run for a while to ensure you're adjusting it under actual operating conditions.

Tighten the idle adjustment screw. Turn the screw clockwise by half a turn and listen to the engine. Most user manuals specify an ideal idle speed, but you can adjust it to your preference for a faster or slower idle. Check the user manual for exact figures and use a tachometer to guide you during the adjustment process.

Listen for any unusual engine sounds and adjust if necessary. The engine will take about 30 seconds to adjust to the changes you've made, so avoid making quick or excessive adjustments. Turn the screw gently and listen carefully to the engine’s response.

Reinstall the air filter and complete the process. Once you've achieved the ideal idle speed according to the specified parameters or personal preference, turn off the engine and replace the air filter to finish the job.