Lean burn combustion is currently a preferred technology to meet the future low emission requirements faced by aero gas turbines. Previous work has shown that the increased air mass flow and size of lean burn fuel injector alters the necessary redistribution of the airflow leaving the high-pressure compressor. This can lead to flow field non-uniformities in the feed to combustor annuli and the fuel injectors which have the potential to impact the overall performance of the combustion system. This paper presents a systematic assessment of the effect of several aerodynamic parameters on the air flow feed to the fuel injectors and the external combustion system aerodynamics for a generic lean burn system. This includes the effect of changes to the flow splits between various combustor cooling features and annulus flows and the effect of a biased compressor exit profile. Flow field data are generated using an isothermal RANS CFD model which is validated against test rig data. The data show that changes in the flow split between the annuli modified the flow uniformity and loss to both the combustor annuli and the fuel injector feed. Changes in the compressor exit profile have a larger effect introducing more notable variations in both flow uniformity and loss. Changes to the angle of the flame tube did not greatly affect the pre-diffuser but did modify annulus loss. Further analysis showed that changes to the combustor annulus flow split, compressor exit profile and flame tube angle modified the location, at compressor exit, of the flow captured by the annuli or each fuel injector passage. The loss to each of these depends on the flow quality (total pressure and uniformity) and from the source more than the flow uniformity delivered.