well sometimes the tip of the needle jet busts off in the carb delivery orifice and your fubar. I am assuming these have the idle fuel adjuster needle on the bottom? take the needle out and blow through it there. In general there is a drilling from the air box side, one of those 2 holes near the bottom inside, which runs back to the drilling which goes into the float cavity, and has the jet threaded into it. then it goes back and meets a drilling into the intake side just in front of the throttle plate on the bottom. directly below this is the idle jet which opens or closes the amount of fuel. start by comparing the needle jets and see if any are missing the tip. And if they aren't, well probably the jet itself is plugged, as those idle passages are much larger then the jets or the output orifice.