The best way to accurately set idle mixture is to use an emission-test machine to read hydrocarbon (HC) and carbon monoxide (CO) levels. As you move the idle-mixture screws, you’ll see some very dramatic changes in HC and CO readings. The ideal idle-mixture setting will minimize both the HC and CO. HC readings are expressed in parts per million (ppm) while CO is given as a percentage. One fact that is generally overlooked is that CO can be equated to air/fuel ratio. For example, 0.01 percent CO is equivalent to 14.7:1 air/fuel and 0.38 percent is equivalent to a 13:1 air/fuel ratio.

This procedure also works for engines with lumpy camshafts. One potential pitfall is that big cams often idle at vacuum levels below 8 inches. These engines also require large throttle openings to allow air in for the engine to idle. This uncovers the idle transfer slot, which can cause off-idle stumble problems. The only way to spot this problem is to remove the carburetor and see if the throttle blades uncover more than 0.040 inch of the idle transfer slot (you can use a spark-plug-gap wire gauge to measure this). If too much of the slot is uncovered, the best fix is to drill a 1/16-inch hole in the leading-edge side of the throttle blade and, if necessary, increase the size by 1/32-inch increments. This will allow more air past the throttle blades, which will allow closing the throttle blade slightly so the transfer slot is only barely exposed by the throttle blade. Be careful not to drill these holes too large or the throttle blades will need to be fully closed to generate the proper idle speed. Unfortunately, this can also create an off-idle flat spot. 

Float level can also affect idle quality. The float level on Holley and Demon carbs is easy to check and modify using the sight plugs on the side of the float bowl. Turn the float-level adjustment screw (arrow) clockwise to lower the float level and counterclockwise to raise it.