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.
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.
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.
Adjusting the mixture is an imprecise art, requiring that you know your engine well and listen closely. 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. Use a scanner to check the air-fuel mixture of the vehicle to help you get the right adjustment. [5] X Expert Source Hovig ManouchekianAuto Repair & Design Specialist Expert Interview. 23 February 2021.
If you need to adjust the idle speed as well, wait to put the air filter back on until you’re finished.
