My '86 300E had been idling reasonably until I installed a new cold start valve in attemp to cure a hard starting problem. Prior to this, the mixture had been tinkered with and probably wasn't right.

Foolishly, after installing the new CS valve, I started tweeking the mixture without going through the formal diagnostic procedure, making the idle problem worse. Now it's running worse than ever and the mixture's obvious rich (more so than before).

I've read all the posts about using a Sears DMM duty cycle feature to set the mixture, but haven't been making much headway. Here's what I've got so far.

Key on, ignition off, O2 sensor connected: 97.61% duty cycle
Key on, ignition off, 02 sensor disconnected: 97.61% duty cycle

Voltage readings using pins 2 & 3 were around 6.85. Turning the mixture screw 1/4 turn clockwise made no difference in the voltage.

With the car warmed up, the O2 sensor voltage was all over the place.

What am I doing wrong?