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This study investigates the relationship between intake port geometry, flame position and stability, and combustion rate and emissions in an air-cooled four-stroke engine where three intake ports of differing geometry have been tested. In particular, the production intake port geometry, and directed and helical intake port geometries were tested. It was a specific intent of this study to investigate the interaction between inlet port geometry, equivalence ratio and ignition timing, without interference from mixture preparation effects. Thus all tests were performed using a homogeneous mixture of propane and air. Significant differences in combustion stability, flame position and stability, burn rate and emissions were observed. For example, the flow induced by the helical port, which should be characterized by a dominant swirl motion, resulted in stabile, asymmetric flames at many of the operating conditions studied.