The lack of clearly identified constraints for ignition and flame propagation has hindered understanding the processes which limit lean operation in spark ignition engines. This experimental study explores flame initiation and flame propagation as limits of lean operation in engines.In separate tests conducted in a single-cylinder CFR (cooperative fuel research) engine, the spark timing was either advanced or retarded from MBT* in order to determine the ignition-limit or partial-burn-limit spark timings, respectively. These two limiting spark timings were found to converge at lean mixtures. At the MBT lean misfire limit, the ignition-limit, and the partial-burn-limit spark timing lines converged. Apparently flame initiation as well as flame propagation considerations constrain lean operation. The effects of engine and ignition system-related variables on the ignition and partial-burn limits are presented and discussed.