what is good oxygen sensor reading

[GRUMPYNOMO]

I have used an O2 sensor in a carburetted engine (MGB with 2 HS6s, cross flow
head, kent cam and headers).  The sensor will never settle out at a mid point
it will always flip flop to one extreme or another and is very sensitive.  If
you lean out the mix you will see the 900mv slope down (I'm using a graphing
scope actually a Fluke 98 series II) to 850 then 800 then it will flop to
around 200mv but you will never get a mixture set that will settle the sensor
out at a mid point. It is not the nature of the sensor to do that.  When you
go drive, you get a chance to watch the SUs richen the mix under load and lean
the mix on overrun.  It is an excellent tool to check the needle profile and
experiment with the viscosity of dashpot oil.