[W126 Coupe] idles rough when properly lean

[Shayegan, Richard]

While my mechanic had the car in to replace the ball joint, I had him look into why my plugs were so fouled up. Turns out the fuel mixture as I had set to get a smooth idle was rich, but setting it lean makes the car idle rough. Any ideas? He said the ignition and fuel systems check out ok, so he was thinking air flow sensor. If it is air flow sensor, anything I can do short of replacing it to get things running right? I have a can of BG 44K lying around, even though the fuel system checks out throwing that in couldn't hurt might help, right?
Richard