This investigation aimed to design a venturi-type biogas mixer to enable small capacity spark ignition engines to operate independently using biogas as fuel. A Honda GX-200 engine was used in the present investigation due to its extensive use in agricultural, commercial, and household applications in the Indian states. The initial design of the biogas mixer was developed using fluid mechanics principles and existing techniques from prior studies. ANSYS workbench 16.2 was used for the computational simulations, refining the design of biogas mixer and to assess the uniformity of air–fuel mixing. After determining the optimal dimensions, the mixer was fabricated and integrated into the engine’s intake system alongside the original carburetor, enabling operation with either biogas or gasoline. Performance evaluation revealed that the retrofitted Honda GX-200 engine achieved a peak brake torque of 11.9 N-m and a maximum brake power of 2.9 kW at 2347 rpm under wide-open throttle conditions.