I'd like to take advantage of the collective wisdom = of The List.  I need some basics on the operation of the digifant = system.  I too have had problems with the lumpy idle, stalling etc.  I have = replaced everything except the AFM (which checks OK with a cheap digi = multi-meter) and the computer. I have messed repeatedly with the  "don't even think = about touching this" , mixture screw. I finally decided to experiment on the = AFM with the adjustments that are found on the "wiper" under the small plastic cover.  Tried changing spring tension.  Looser (allowing wiper = to move more for a given volume of air) helped some.  Changing the position = of the wiper contact relative to the air flow vane (richer) helped alot, = **but** only with the 02 sensor disconnected.  When I reconnect the 02 sensor, everything returns to crap.  The easy answer is that I need to = replace the 02 sensor right?  Did that 2 years ago.  Ran like crap before = and after the new sensor.  While doing my experimentation, I have = connected the multimeter to the 02 sensor output (while sensor is disconnected from = the computer). It is working.  All of the basics are correct: no = vacuum leaks, good ignition, new fuel filter, idle stabilizer, = etc.  The static timing is advanced @ 5 deg.  Makes more power this way, = and does not seem to effect anything negatively.    The engine = really only wants to run well when the 02 sensor shows that it's running very rich: = .80 to .90 mv.  At this setting, it really does run well.  Steady = idle, no stumble on clutch release, no popping and kicking on decel, and gobs of = power at cruising speed.  The question then is: why can I only get this = thing to run right when the injection is apparently too rich?  Is this what = VW intended?  Is this the only way that it would pass emissions specs = or get acceptable mileage?   Does the 02 sensor provide input to the = computer at all operating speeds/loads or just at idle?  Does the C0 = adjustment screw affect the entire "map" of operation or just at specific = speed/load or when the 02 sensor is out of the loop?  I am very happy with the = way our wbx is running now, but this setup is not right.  Why?