This air/fuel monitor consists of an O2 sensor and a calibrated electronic display. Simply install the O2 sensor in the exhaust pipe, and then connect the display. When the engine is running, the actual air/fuel ratio is continuously displayed. The monitor indicates air/fuel ratios from approximately 12 to 15. Use it to guide carburetor adjustment and jetting or to improve ignition spark advance curves. Not recommended for use with leaded fuels.