What kind of carb? Check the simple things first - vacuum lines, idle adjustment screw in place, all linkage in the right relationship, etc. You didn't run out of gas recently did you? I filled a carb with sediment crap many years ago and it wouldn't idle at all but still ran OK at WOT or even in heavy mid-range. Also, what about your fuel pump? If it's failing it could starve the carb at idle, but then pick up some at higher rpm's? Grasping at straws here....