As I remember from the early days of emission control the vacuum retard at idle was to combat the high CO & HCs that occur when you close the throttle abrubtly. It was a primitive attempt at cleaning up emissions. I remember that you could also "pass" an emissions tailpipe test[before the rollers were used] by retarding the timing. It would run like sh... but stillpass the sniffer. After the test you reset the timing to make it run again.

Mike M.