why does the A/F ratio reads leaner when misfires occurs? today i was tuning @ 26lbs and all was good. the A/F ratio was between 12.1 - 11.7. then boom, on the next run it starts to break and the A/F ratio went from 13.0 - 12.8. WTF!!

 

Because the O2 sensor is measuring % of oxygen in the air...if there is a misfire, the fuel isnt burnt and neither is the normal amount of oxygen that would have been so the sensor reads a higher % of oxygen which relates to a leaner mixture.