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The wider flammability limit of lean natural gas-air mixtures offers potential for operating spark ignition engines on lean air-to-fuel ratios. However, at very lean equivalence ratios, the development of the initial flame and its subsequent propagation becomes highly sensitive to physical and chemical state of the mixture. This in turn, can adversely affect engine performance, particularly the cyclic variation in the combustion process. This paper discusses the effects of lean-burn operation on the flame development durations and the cycle-by-cycle variations in a natural gas fuel injected engine. The study was conducted on a 8-cylinder, 4.6 liter, spark-ignited engine. A data acquisition system is used to acquire 300 consecutive in-cylinder pressure cycles. A heat release model was used to estimate the initial flame development time and the rapid burn duration.