the idle air bleeds allow air to enter the idle circuit. so the more you open the idle mix screw the more mixture you pull thru the it. with the bleed plugged its only pulling fuel, hence the reason you can still kill the motor. but when the bleed is open you get a LEANER mixture.
what you need to do is pull the primary side (or both if 4 corner idle) float bowl and block off and blow air thru the passages. then spray some carb cleaner. i like to use a flashlight and look thru the passages (flashlight up against the block side of the body looking thru the bleeds) no light means its plugged