Systems and methods for continuous automatic adjustment of a control set point of an air-fuel ratio controller of a rich burn engine so as to maintain emissions within a desired range. In an embodiment, a NOx sensor is placed in the exhaust outlet from an exhaust catalyst and the NOx exhaust sensor output is continuously monitored while adjusting the control set point between rich and lean in order to minimize the output from the NOx sensor. Corrections may be continuously made to the set point during operation of the engine to compensate for changes in environmental conditions, engine loads, and other factors.