As I said I have experienced this where the timing being low caused an overly rich AFR. If there isn't enough timing to burn the fuel then your AFRs are going to richen up especially when load increases in the 5k-redline area. Of course the degree of timing that can cause such situations varies from car to car.
Don't get me wrong I am not saying you are incorrect, however I am just stating what I have experienced as well as citing the reasons.
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Originally Posted by
BenFenner
I think you have that backwards. Retarded timing produces what appears to be "lean" readings because there is still plenty of unconsumed oxygen entering the exhaust. Highly advanced timing shows results that appear to be "rich" because much more of the oxygen is consumed before entering the exhaust.
This is all assuming the same exact fueling.