How to use a vacuum gauge on a carbureted engine to diagnose problems like a sticky valve, ignition component problems or too much advanced timing, a dirty air filter, vacuum leaks, worn piston rings, improper cam timing, rich or lean mixture, and more. More information is located on Holley's website: http://forums.holley.com/entry.php?429-How-To-Adjust-The-Idle-Mixture-On-Holley-Carbs