or it's actually running lean and the resulting misfires cause unburned fuel to enter the exhaust? really lean input can look a lot like really rich, as the unburned fuel can pool up, not fire, then burn rich/inefficient on subsequent ignition. i always have these guys say their car is running rich due to exhaust smell, i end up removing a ton of fuel to fix it....

experiment. go 20% richer and see what happens, then 15% leaner. you'll quickly figure out what direction to go in.