I think there is a misconception of what the basic mechanical mixture adjustment does. It ONLY affects the mechanically derived idle mixture based on air flow plate deflection when the car is in open loop operation during cold start and warmup. Once the O2 sensor gets up to 300C and the engine goes into closed loop operation (typically at around 60C coolant temperature), the O2 sensor and electronics will maintain a stoichimetric mixture at all conditions except WOT as long as the basic mixture adjustment is not so far out of wack that the control system has insufficient authority to maintain stoichiometry.

A slighly "rich" mixture at idle, as indicated by the duty cycle <50%, will aid cold start and warmup driveability, but will not have a signficant impact on fuel economy unless your average trip length is less than a couple of miles.

I measured my duty cycle in the range of 40-45 percent at idle and 50-55 at 2500 revs, no load, and my car has excellent cold start and warm-up characteristics, so I think the duty cycle is just about perfect.

What are your measured duty cycles at idle and 2500? A difference in duty cycle of no more than ten percent is probably considered "ideal", but there is no actual specification as long as the mixture can be maintained at stoic. by the control system during closed loop operation, which would likely be well over 20 percent difference, however if the idle duty cycle is over 50 percent, cold start and warmup may be poor due to leaness.

Another thing to consider is that if you have oxgenated fuel during the winter and non-oxygenated fuel in the summer you will get slightly different duty cycle readings with summer and winter fuel. Say you achieved a 50 percent idle reading on non-oxygenated summer fuel. The duty cycle on winter fuel would increase because the O2 sensor will detect a "lean condition" because of the extra oxygen and will have to run, on average, a little richer to maintain an average low O2 content in the exhaust.

Duke