Step-by-Step Methods for Adjusting a Carburetor

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posted on 13 years ago — updated on 1 second ago

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You don’t need to be a professional engineer to adjust the carburetor in your vehicle. If you notice your engine is running a bit rough, it’s important that you go in and adjust the air and fuel mixture and find the correct idling speed of your carburetor. Fortunately, this can all be done in just a few simple steps, and it requires no special tools or equipment. Keep reading to learn how to adjust your carburetor so your engine runs smooth and stays in great shape for a long time to come.

Things You Should Know

Start your engine and let it warm up to its normal operating temperatures.
Listen to your engine. If the engine runs rich, there will be no unusual sounds, and if it runs lean, it will ping at higher RPMs (revolutions per minute).
Bring your engine to a lean mixture by turning its screws a quarter-turn at a time, counter-clockwise. Then, slowly bring them back to equal.
Tighten the idle adjustment screw to increase idling speed and loosen the screw to decrease idling speed.

Adjusting the Air and Fuel Mixture

Locate the air filter and remove it. On most cars, you'll need to remove the air filter to expose the carburetor and adjust it. Open the hood and make sure the engine is off before locating the air filter and removing the assembly. Unscrew the wing-nut and any other connectors, and then remove the air filter entirely. Depending on your make and model and the engine type, the air filter could be in any number of different places on the engine. Consult the owner's manual or the shop guide for your vehicle. On most carbureted cars, the air cleaner housing is attached directly to the carburetor.

Find the adjustment screws on the front of the carburetor. These often look like flat-head screws. There should be two screws on the front of the carburetor, which are used to adjust the air and fuel mixture. These often look like flat-head screws, and you can use a screwdriver to turn them, adjusting the amount of fuel and air mixing in the carb. Some carburetors, such as the Quadrajet found in most GM vehicles, have a special screw and require a specific adjusting tool. The Quadrajet uses a Double "D" carburetor adjusting tool. Other carburetors may have a 4 corner idle mix adjustment and will therefore have 4 idle mixture screws.

Start the engine and let it warm to normal operating temperature. Check the temperature gauge to know when it's at the appropriate running temperature. Then, listen to the sound of the engine to get some sense of the adjustments that need to be made. An engine that runs lean will ping at higher RPM (revolutions per minute) when the throttle is open, as if you were flooding a gear. In this case, more gas needs to be added to the mixture. An engine that runs rich won't necessarily make a change in sound, but you'll be able to smell it. In that case, reduce the amount of gas. An engine that runs too rich at idle will cause the plugs to be fuel-fouled, leading to a vehicle that is harder to start cold.

Adjust both screws equally and find the right mixture. Adjusting the carburetor is a lot like tuning a guitar. Use a screwdriver to turn the screws equally, smoothly, and slowly until you find the sweet spot. Regardless of whether or not the engine is running too rich or too lean, bring it down to a very lean mixture by turning both screws a quarter-turn at a time, counter-clockwise, then slowly bringing them back up to an equal and smooth mixture. Listen to the engine to know when you’ve found the right balance. The engine should purr smoothly once you’ve found the right mixture. Any raggedness or rattling is a sign of too lean a mixture. Keep turning until you find the sweet spot. Use a scanner to check the air-fuel mixture of the vehicle to help you get the right adjustment.

Replace the air filter assembly. When you've got the carburetor adjusted, put the air filter back on and you're ready to roll. If you need to adjust the idle speed as well, wait to put the air filter back on until you're finished.

Adjusting the Idle Speed

Remove the air filter to access the carburetor. Make sure the engine is off before locating the air filter and taking it out. Unscrew the wing-nut and any other connectors, and then remove the air filter entirely. Consult the owner’s manual or shop guide to find out the exact location of your air filter, since it can vary depending on your make and model and engine type. On most carbureted cars, the air cleaner housing is attached directly to the carburetor.

Find the idle adjustment screw. There is an idle speed screw that adjusts the opening of the throttle plate, and an idle mixture screw that limits the flow of fuel at idle. You want to adjust this second screw. As always, consult the owner's manual or shop guide for your make and model, if you can't find the screws.

Start the engine and let it warm to running temperature. Just as you did with the fuel/air mixture, let the engine warm up some to make sure you're adjusting at actual running condition.

Turn the idle adjustment screw half a turn clockwise to tighten. Turn the screw clockwise, no more than a half-turn, and listen to the engine. Most owner's manuals will have an optimum speed to set the idle, though you have some wiggle room if you prefer it higher or lower. Consult you owner's manual for the number and consult the tachometer as you adjust.

Listen to the car's engine for sounds of roughness and readjust if necessary. It should take about 30 seconds for the engine to adjust to the change you made, so wait a bit before making another adjustment. Make slow turns and listen closely to the response.

Replace the air filter and finish the job. When you've got the idle adjusted to the proper specifications, or to your own preferences, kill the engine and replace the air filter to finish the job.