Set Idle Speed Correctly
Most car crafters don't realize how important the idle mixture setting is for a carbureted engine. And most don't know that once the throttle is opened, the idle system continues to deliver fuel at part-throttle and that for the majority of light-throttle highway cruise, your engine is running on the idle circuit. Based on that, you can begin to see why setting ideal idle mixture is so important. Let's start with an engine that is fully warmed up, the choke is completely off, and the float level is set correctly. The following procedure applies to most all Holley, Demon, Carter, Edelbrock, and Rochester carburetors.
First, with the engine off, lightly turn each idle mixture screw clockwise and count the total number of turns. Then return each screw to its original position. If the idle screws are not the same side to side, make the necessary changes so they are the same. If you have a vacuum gauge, it will help, as will an accurate tachometer. Start the engine and allow the idle to stabilize. Start by turning one of the idle mixture screws in (clockwise) by 1/8 turn and note any change in engine speed or vacuum. This change will lean out the idle air/fuel mixture slightly. If engine speed or vacuum increases, make this same change to the other idle mixture screw. Continue this process as long as engine speed or vacuum increases. Always turn both screws the same amount and allow the engine to stabilize before making further changes. At some point, the idle speed or vacuum will begin to drop. When this occurs, turn both mixture screws counterclockwise back to the highest engine speed and vacuum reading. This establishes the leanest possible idle mixture setting that avoids lean misfire.
If the initial clockwise movement of the idle mixture screw reduces engine speed, then the procedure is to richen the idle mixture by turning the idle mixture screw counterclockwise in even steps until you find the highest engine idle speed and vacuum. If engine speed increases roughly more than 200 rpm past your ideal engine idle speed, take the time to adjust the idle speed back to its base speed.
If you find only one idle mixture screw is responsive, the idle circuit may be blocked due to dirt or possibly a plugged idle air bleed located in the top of the carburetor. Sometimes squirting a little carb cleaner through the air bleed will solve the problem. Also, if the engine is unresponsive to idle mixture changes and the idle hunts or moves around, it's a classic symptom of a vacuum leak. Loose intake manifold or carburetor bolts are common culprits, as is an uncovered vacuum line or a leaking intake manifold gasket. Also make sure all the carburetor bolts are snug, as this can also cause erratic idle performance. If you suspect a vacuum leak, it must be repaired before attempting to properly set idle mixture.
The most accurate way to set idle mixture is with a smog machine that indicates HC (unburned hydrocarbons) and CO (carbon monoxide) or with a wide-band oxygen sensor that outputs air/fuel ratio. With a smog machine, you can instantly see the HC level increase as soon as a too-lean idle mixture creates a lean misfire. This will occur long before you can feel the misfire in the engine. Shoot for the lowest HC and CO numbers you can generate. If you have access to an air/fuel ratio meter, shoot for the leanest air/fuel ratio that retains crisp throttle response with no off-idle hesitation. After setting the idle mixture, drive the car to evaluate the changes. If the engine surges or stumbles, you will have to richen the idle mixture slightly to establish good throttle response. Part-throttle surge is usually the result of an off-idle transition circuit that is too lean. One urban myth is that a lean idle air/fuel ratio will make the engine run hot. This is false. In fact, a leaner idle allows the engine to run cooler. That's not a misprint.
 This is where the idle mixture...  This is where the idle mixture screws are located on Holley and Demon carbs. Many Holley and most Demon carbs now come with four-corner idle, so look for idle mixture screws in the secondary metering block as well. |  On a Rochester Q-jet, the...  On a Rochester Q-jet, the idle mixture screws are located in the front of the carb at the base. |  The idle mixture screws are...  The idle mixture screws are easy to access at the front of Carter and Edelbrock four-barrel carbs. |