Adjusting the Air and Fuel Mixture
1
Locate the air filter and remove it. On most cars, you'll need to remove the air filter to expose to 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 kind of engine in the vehicle, 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.
2
Find the adjustment screws on the front of the carburetor. There should be two screws on the front of the carburetor, which are used to adjust the air and fuel mixture.
Often these 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 (4 idle mixture screws).
3
Start the engine and let it warm to normal operating temperature. Check the temperature gauge to know when it's at the appropriate running temp, and 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, when the throttle is open, as if you were flooding a gear. More gas needs 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. Bring the gas down some.[1] 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 or other stringed instrument. You want 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.
- Adjusting the mixture is an imprecise art, requiring that you know your engine well and listen close. Bring both screws up slowly and listen until the engine purrs smoothly. Any raggedness or rattling is a sign of too lean a mixture. Keep turning until you find the sweet spot.
5
Replace the air filter assembly. When you've got the carb 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.[2]
Tips
- If your car is equipped with a tachometer, you can use this as a tool to adjust idling speed (revolutions per minute or RPMs). Check your owner's manual for the correct RPMs.
- Tightening the idle adjustment screw increases idling speed while loosening the screw decreases idling speed.
- If after adjusting the idle mechanism, the engine does not run smoothly, go back to making air and fuel adjustments and repeat the steps for both air and fuel and idling adjustments.
- There is a difference in idle speed with the car in neutral/park versus in gear for an automatic car. Do not adjust idle speed with the car in gear unless there is someone in the driver's seat with their foot on the brake.
- There are some cars that have multiple carburetors, i.e. European and Datsun, that must also be balanced for air flow before adjusting.
Warnings
- Remember that when you are working on a carburetor, you are working with a fuel source. Take all necessary precautions for working around gasoline.
- Keep in mind you are also working on a running motor. You can seriously injure yourself or damage your engine if you are not careful. Work slowly and thoughtfully and make sure your clothes don't have any hanging strings or cords that could catch in the engine.
