In piston engines at fuel/air mixtures above 0.095, the expected relationship between fuel/air mixture and fuel flow appears to break down. Here is a plot of ENG_FUEL_FLOW_GPH vs. RECIP_MIXTURE_RATIO (data collected by simconnect) for the Flysimware Chancellor:
As fuel/air ratio rises above 0.095, EGT continues to fall as expected, but the fuel flow abruptly stops rising and begins a gradual decrease. The pattern becomes more exaggerated at excessively rich mixtures. At around 0.110 fuel/air ratio, the fuel flow decreases more rapidly, and above 0.120, the decrease is even faster. Here is a plot from the stock G36 Bonanza:
It appears that fuel flow is serving as some kind of proxy for engine power or perhaps fuel burn rate at very rich mixtures. Why does this happen? Is there a way to tune fuel flow for more realistic values at high mixture ratios?