Measurements of cylinder pressure and flame travel velocity have been obtained in a single cylinder engine with two arrangements of port geometry and with mixture equivalence ratios from 0.68 to 0.9. They are complemented by photographs of the flame development and measurements of local velocity. The investigation compares the combustion processes in terms of the maximum pressure, flame speed and in-cylinder flow velocity without and with an intake shroud which increased both the tumble and swirl ratios. The extent to which residual burned gas retarded the combustion rate and increased cyclic variability are quantified. The photographic studies confirm the dominant effect of the swirling flow on flame propagation and deviations of the flame kernel from spherical as the air-fuel ratio is increased, with much higher probability of influence of velocity fluctuations. Port injection of the gaseous fuel is shown to lead to results identical to those with premixed fuel and air introduced into the manifold.