The duty cycle is a synthesized signal that measures the relative amount of time that the lambda system spends adding a rich or lean bias to the basic mixture provided by the air flow sensor and mechanical section of the fuel distributor and is used to make any necessary mechanical mixture adjustments to keep the lambda system near the mid point of its control authority range.

It's essentially a measure of the mechanical mixture bias. If the duty cycle is 50 percent it means the mechanical mixture is spot on stoichiometric, but the mechanical mixture adjustment is not accurate enough to maintain an exact mixture, so the duty cycle will vary slightly as the lambda system corrects the mixture to stoichiometric with finer granularity and greater accuracy.

The other criterion is that you want the average duty cycle at idle to be within 10 percent of the average at 2000 RPM no load. I prefer a slightly less than 50 percent duty cycle at idle as this indicates a slightly rich bias of the basic mechanically derived mixture, which will aid cold starting and warmup driveability. When I checked my duty cycle it was in the range of 40-45 percent at idle and 50-55 percent at 2000, so I made no adjustments, and the cold start characteristics are excellent.

If you have a scope, you can eavesdrop on the actual O2 sensor output by pulling the O2 sensor connector out just enough to get a scope test lead attached with the other scope test lead to ground. The wave form should jump quickly between about 0.15 and 0.88 volt and back with a frequency of about 1-3 Hz at idle.

Duke