MC CIS oxygen sensor question

[Cody Forbes]

Had my white 5ktq in the shop tonight for an oil change and check over. I decided to play with the fuel mix a bit as mileage has gone down recently (yes, I know it has a vacuum leak, the throttle body shaft seals leak when on the smoke machine). So I unhook the oxygen sensor, adjust the mix by ear and make a noticeable improvement in idle quality with about a quarter turn clockwise. Funny thing is though that when I plug in the oxygen sensor again the idle quality deteriorates. Is this expected or does it sound like oxygen sensor time? I know it's got at least 6 on it (my history with the car), likely many more than that.

I'm so used to my black EFI 5ktq that I am forgetting all the CIS tidbits!! 

-Cody (mobile)