Abstract-The amount of exhaust gas recirculation (EGR) which a single-cylinder engine tolerated without misfire increased when the duration of the spark was increased. Also, when minimum spark advance for best torque was maintained, engine efficiency improved as EGR increased. In addition to misfire, two other engine combustion problems were identified: partial combustion, and excessively slow combustion. Low flame speeds associated with high EGR probably caused these other problems, which could be prevented with advanced spark timing. Misfire, partial combustion, and excessively slow combustion were detected by monitoring with an oscilloscope the output of a bare wire thermocouple located in the exhaust port of the engine. This simple method could be suitable for quantifying combustion problems in multi-cylinder engines.