Originally Posted by SteveP

Let the engine warm up, then rev the engine up a couple of times to clean out any excess fuel in the crankcase. Put the car up on pit wall (if you're at the track) or on a car stand, then pinch the fuel line. If the engine stalls in less than 6 seconds the low-speed needle is too lean. If it takes longer than 8 seconds, it's too rich. If the mixture is too rich, you may also experience a considerable increase in engine rpm (hence the reason for getting the wheels off the ground), which tells you that the idle speed is too high. So, if the engine runs on for a long time after pinching the fuel line, lean the low-speed needle and reduce the idle speed. You want the engine to run 7-8 seconds after pinching the fuel line, and without a major change in engine idlle speed. It will rev up some, but not a lot, if the low-speed needle and the idle speed are set correctly.