Experiments conducted with a variety of fuels in an optically accessible, compression-ignition engine have revealed a strong correlation between hydrocarbon (HC) emissions and the flame lift-off length, where increasing HC emissions are associated with longer lift-off lengths. The correlation is largely independent of fuel properties and charge-gas O2 mole fraction, but varies with fuel-injection pressure. A transient, one-dimensional jet model was used to investigate three separate mechanisms that could explain the observed impact of lift-off length on HC emissions. Each mechanism is associated with the formation of mixtures that are too lean to support combustion, or “overlean.” First, overlean regions can be formed during the ignition delay. Second, during fuel injection, longer lift-off lengths could increase the mixing of fuel with charge gas upstream of the lift-off length, such that more of the injected fuel becomes overlean. Third, after the end of injection, a region of increased entrainment and mixing upstream of the lift-off length could cause late-injected fuel to become overlean. The model revealed a correlation between lift-off length and overlean regions created during fuel injection that closely matched the experimentally observed trends. HC emissions associated with overlean regions created during the ignition delay and after the end of injection, however, did not correspond to the experimental data.