I whiled away 1.5 afternoons this weekend doing the O2-sensor-as-mixture-
tester trick.  Paid $45 for generic three-wire O2 sensor, $25 for cheapest
Radio Shack DVM.  Wired so I could power the heater leads from the cigarette
lighter, and see the voltage display as I drove.

As I had read posts about the idea, I had pictured sticking the sensor into
the tailpipe (eg on a copper pipe to protect the wiring) but with sensor in
hand I realized this won't work:  pigtail end of sensor must be in outside
air for it to work.  I tapped the sensor into a pipe that is a press fit
into my tailpipe.  

Very educational to watch the display.  "Sweet spot" setting of mixture
(on 77 CIS engine) is very narrow and has nothing to do with best (ie
stoichiometric) setting at idle.  I can't imagine pulling the plugs often
enough to get it right without a tester that can go on the road.  Does this
match the experience of others -- that CIS system is rich at idle in order
to be neither rich nor lean at speed?  Or do I need to install clean plugs
and try again?

B.