The idle mixture adjustment is directly related to your lean/rich at an idle. If it isn't adjusted correctly, many of your other tuning parameters cannot be accurately set either. If you have back fired the engine during your tuning stage, you could have popped the diaphragm in the power valve, causing an overrich condition, and it will need to be replaced before proceeding. With the engine idling, look down inside the top of the carb and see if you see gas pouring profusely out of the squirter.
