The conversion of existing internal combustion engines to natural-gas operation can reduce U.S. dependence on petroleum imports and curtail engine-out emissions. In this study, a diesel engine with a 13.3 compression ratio was modified to natural-gas spark-ignited operation by replacing the original diesel injector with a high-energy spark plug and by fumigating fuel inside the intake manifold. The goal of this research was to investigate the combustion process inside the flat-head and bowl-in-piston chamber of such retrofitted engine when operated at different spark timings, mixture equivalence ratios, and engine speeds. The results indicated that advanced spark timing, a lower equivalence ratio, and a higher speed operation increased the ignition lag and made it more difficult to initiate the combustion process. Further, advanced spark timing, a larger equivalence ratio, and a lower speed operation accelerated the flame propagation process inside the piston bowl and advanced the start of the burn inside the squish. However, such conditions increased the burning duration inside the squish due to more fuel being trapped inside the squish volume and the smaller squish height during combustion. As a result, the end of combustion was almost the same despite the change in the operating conditions. In addition, the reliable ignition, stable combustion, and the lack of knocking showed promise for the application of natural-gas lean-burn spark-ignition operation in the heavy-duty transportation.