Tools: Small straight screwdriver; Vacuum gauge or low-speed tachometer
Little things can make a big difference in how an engine performs. Street engines spend a lot of time at idle and off-idle speeds, so on carbureted engines it's important that the idle mixture be adjusted properly. This improves driveability, minimizes rough idle, and can provide cleaner emissions and better fuel economy. Here's a quick look at how to set idle mixture on a stock or mildly modified engine.
All engines require an acceptable air/fuel ratio to run properly, even at idle. Stock or mildly tweaked engines generally idle at 16-18 inches of intake-manifold vacuum between 750 and 900 rpm. Carburetors equipped with adjustable mixture screws allow the tuner to change the amount of fuel delivered to the engine, creating an ideal idle mixture.
On most carburetors, turning the idle mixture screw clockwise leans the idle mixture. Turning the screws counterclockwise richens the mixture. If there's a vacuum leak, it must be repaired before the idle mixture can be properly adjusted.
All stock engines specify a certain idle speed, but generally 800-850 rpm works well. Properly setting the idle mixture also creates the highest manifold vacuum, so a vacuum gauge makes the task easier.
The best backyard procedure for setting idle mixture is the idle-speed-drop method. First, connect the vacuum gauge to the manifold. Adjust the idle mixture richer or leaner until the highest vacuum is read. Once that's attained, turn the idle mixture screws leaner until the rpm begins to drop, then turn the screws richer a slight amount. The ideal idle mixture has now been set.
If you don't have a vacuum gauge, use a low-speed tachometer. The tach should have a low-scale readout that's sensitive to increments of at least 50 rpm. Use the tach to help you adjust the idle-mixture screws evenly to create the highest idle speed. If the idle speed exceeds 950 rpm, use the idle speed screw to slow down the engine, then readjust the idle mixture.
This isn't a difficult procedure, but it can be tough if the engine has a big cam and low manifold vacuum. If the carburetor requires a large throttle-blade opening, modifications will be necessary. Otherwise, this is a simple procedure that anyone can do in a matter of minutes.
The idle circuit illustration shows how fuel is pulled from the main well past an idle-feed restriction and across an idle air-bleed to mix air with the fuel. The air and fuel is then directed past the idle mixture screw before being delivered to the intake manifold. | 
The best way to set idle mixture is with a vacuum gauge. Shoot for the highest vacuum level at a certain rpm (for example, 850). If you don't have a vacuum gauge, use a low-speed tachometer that reads small rpm changes. |
The location of the idle mixture screws varies with each carburetor. Most Holley four-barrel carbs locate the idle mixture screws on the metering block. | 
Rochester Quadrajets position the screws low in the front (B). |
Most Carter four-barrels also locate the mixture screws in front (C). | |