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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. 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). 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. 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. 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. 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.
Locate the air filter and remove it. Find the adjustment screws on the front of the carburetor. Start the engine and let it warm to normal operating temperature. Adjust both screws equally and find the right mixture. Replace the air filter assembly.