Two-valve pent-roof type combustion chamber was employed for four stroke cycle small utility engine. The preliminary performance test showed the higher output was obtained by pent-roof type combustion chamber than the conventional bathtub type combustion chamber. CFD analysis and PIV flow visualization proved that the tumble flow is dominant in pent-roof type combustion chamber. From the investigation of the relationship between the tumble ratio and the engine performance by combustion analysis and cycle simulation, it was found out that the tumble ratio from 0.8 to 1.6 is optimal for engine output. By applying the pent-roof type combustion chamber, the developed engines achieved 7-12% increase in output, 8-10% reduction in BSFC. As for the emission, the developed engines can satisfy the CARB regulations.