This paper focuses on the effect of fuel-air ratio on the performance and cyclic variations of single cylinder hydrogen and gasoline fueled spark ignition engine. The hydrogen fueled engine was operated at the wide-open throttle (WOT) condition, while the engine fuelled with gasoline was operated with conventional carburetor mode by varying the throttle opening to vary equivalence ratio. All the experiments were conducted at a constant engine speed of 1600 rpm. The spark timing was set to minimum advance for best torque. As a result, it was found that there is about 29% reduction in the maximum power output of the engine when operating with hydrogen. The equivalence ratio could have been varied in the range of 0.3 < phi < 0.9 for hydrogen and of 0.8 < phi < 1.3 for gasoline. In all combustion periods, cyclic variations for hydrogen engine were much lower than that of gasoline engine. As a consequence, it was found that the engine gave similar output at equivalence ratio of phi = 1.1 for gasoline and phi = 0.6 for hydrogen. However the engine was operated more smoothly when fuelled with hydrogen.