Low nitric oxide (NO) emissions and good fuel economy are obtainable at very lean mixtures. However, unstable operation caused by misfire and erratic combustion prevents present spark ignition engines from being operated very lean. A study was undertaken to understand what causes very lean mixtures to misfire in engines. The effects of mixture preparation, intake airflow, exhaust gas recirculation (simulated by N2 dilution), compression ratio, intake mixture temperature, engine speed, number of spark plugs and spark plug locations were investigated at minimum advance for best torque (MBT) spark timing in single-cylinder engines. Propane and isooctane were the fuels used.Results showed that leaner operation was possible with improved mixture preparation, increased airflow, decreased nitrogen (N2) dilution, increased compression ratio, increased mixture temperature, decreased engine speed, more central spark location, and multiple spark plugs. The dominant factors responsible for these effects were mixture homogeneity, charge dilution, and charge temperature. In most cases, at the lean limit the combustion duration (in crank angle degrees) reached a limiting value. The significance of these findings together with the exhaust emission and fuel consumption results are presented and discussed.