I know that when I adjusted my lambda the idle got smoother. The closer I adjusted to 50% duty cycle the better the idle got. I don't know exactly why.

Here, maybe Steve B's article will help a little...
http://www.continentalimports.com/ser_ic20134.html


Interesting info though. That makes two very interesting things to do with idle in these cars that I have learnt from this forum today...