Unlike carbs, there is no idle circuit. There is really only the main circuit which is modified by time, temp and vacuum through changes in the contrlo pressure. Later versions also have a feedback from an O2 sensor which modifies the control pressure we spoke of. It may be running rich across the band which may only show up at slower speeds. The WUR may lag the light going off in it's response.
There are also air bypass valves which can fail, but JRV could address that better than me.
JRV would be the 'Man' on this.
