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How to Adjust the Carburetor on Cars

by Contributing Writer; Updated June 12, 2017

The carburetor on a two-stroke engine has a mixture setting for the idle speed and a mixture setting for the high speed. Adjusting the carburetor for this type of engine is part of the process that is known as tuning the engine. Tuning a two-stroke engine will keep it running at peak performance. You can adjust the mixture settings on the carburetor by turning two screws that are located on the carburetor.

Under The Hood:

 How Do I Adjust the Carburetor on a 1965 Mustang?

Position wheel chocks in front of both rear tires.

Apply the parking brake.

Turn the engine on and turn the air conditioning off if the Mustang is equipped with air conditioning.

Place the transmission in "Drive" if equipped with an automatic.

Turn the idle mixture screw on the driver's side of the carburetor either into or out of the carburetor with a screwdriver until the proper idle speed is reached as indicated by the Mustang's tachometer. Turning the screw will change the idle speed. Turn the screw in either direction and note whether the idle speed increases or decreases. If the desired idle speed is not reached by turning the screw in one direction, turn it in the opposite direction. If the Mustang is equipped with an automatic transmission, the idle speed should be 600 revolutions per minute. If the Mustang is equipped with a manual transmission, the idle speed should be 500 RPM.

Items you will need

  • Wheel chocks

  • Screwdriver

 How to Adjust the Carburetor on a Small Two-Stroke Engine

Locate the two adjustment screws on the carburetor. One screw is for the idle speed or low speed; the other is for the high speed.

Turn both of the screws counterclockwise to back them out. This will set them in the "open" position.

Turn each screw clockwise two full turns. This will be the starting point for adjusting the carburetor.

Start the engine and let it warm up for a minute or two. Adjust the choke lever to help start the engine, if needed.

Turn the screw for the idle speed clockwise until the motor sounds like it is going to stall, then turn it counterclockwise ¼ to ½ turn.

Hold the throttle for the engine wide open to run the engine at high rpm. Turn the screw for the high speed counterclockwise until the engine begins to run smooth, then turn it ¼ turn clockwise.

Items you will need

  • Screwdriver

 How to Adjust a Carburetor on a Slant Six

Remove the air cleaner that mounts directly on top of the carburetor. Examine the base gasket and make sure it is intact and leak-free.

Install the tachometer leads by attaching the negative lead to the negative connection on the ignition coil. Attach the positive lead to the battery positive post. Start the car and read the tachometer.

Adjust the base idle screw on the carburetor and read the tachometer. This screw sets the idle linkage (the linkage that is attached to the accelerator pedal) to the factory idle specifications. It is normally 500 to 550 revolutions per minute (RPM). Adjust the screw in or out until the proper RPM is achieved.

Adjust the idle mixture screw with a screwdriver. This is a small screw at the base of the carburetor. A spring is behind the head of the screw to keep it from vibrating out of place. Turn the screw in to create a lean fuel mixture until the engine starts to misfire. Back the screw out (richen the fuel mixture) until the engine runs smoothly.

Items you will need

  • Screwdriver set

  • Tachmoterer

 How to Adjust a Carburetor in a Corolla

Lift the hood of your Corolla and find the air filter and associated air cartridge. Remove both from the vehicle.

Locate the idle mixture screw and rotate it clockwise until the registering needle hits the seat of the carburetor. Rotate the screw counterclockwise one and a half times. Rotate it counterclockwise until the screw reaches the seat of the emulsion tube if your carburetor has a main jet adjustment attached to it.

Return the air filter and cartridge securely to their original places, but leave the hood up. Enter the driver's seat and restart your vehicle.

Run the Corolla engine at half-throttle for several minutes to make sure it properly warms up. Exit the vehicle and turn the idle mixture screw clockwise one half rotation. Turn it back to its original position after the engine has slowed down.

Adjusting the Choke Linkage on Your Corolla Carburetor

Take out the air filter and cartridge from the engine. Locate the choke lever and switch the setting to "Fast." Loosen the mounting brackets, so the cable casings can move without restraint and tweak them until the engine's choke is shut.

Screw the cable mounting back into place. Check the movement of the control lever until the choke linkage has been adjusted to the appropriate level as directed by your vehicle's user manual.

Close the hood and restart the vehicle's engine. Take your Corolla for a test run to ensure that it's functioning properly.

Items you will need

  • Screwdriver

 How Do I Adjust the Carburetor on a 1967 Ford F-100?

Start the engine and allow it to run until the it reaches its normal operating temperature, as indicated by the temperature gauge in the dashboard.

Pull the rubber vacuum hose from the side of the carburetor body and attach the hose from the vacuum gauge in its place. Adjust the idle mixture screw on the body of the carburetor with a screwdriver until the reading on the vacuum gauge matches the setting specified in a service manual for your carburetor and engine combination. Back out the screw to increase engine speed and vacuum reading, and tighten it to decrease both. Remove the vacuum gauge and reattach the vacuum hose.

Adjust the idle speed of the engine by turning the idle speed screw, which is set into a plate as the base of the throttle linkage. Use a screwdriver to tighten the screw to increase idle speed, and back it out to decrease idle speed. Listen to the sound of the engine as you turn the idle speed screw, no more than a 1/4-turn at a time, in each direction. Adjust the screw until the engine is running the smoothest. If the adjustments don't help, the carburetor needs to be rebuilt.

Items you will need

  • Vacuum Gauge

  • Flat-head Screwdriver

About the Author

This article was written by the It Still Runs team, copy edited and fact checked through a multi-point auditing system, in efforts to ensure our readers only receive the best information. To submit your questions or ideas, or to simply learn more about It Still Runs, contact us.

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