This is an optional REDS Engines 6.5mm Carburetor Venturi. This part is compatible with the REDS R5, R5C, R5T and R5TE engines. The carburetor venturi is used to adjust the airflow that is allowed into the engine. By changing the size of the venturi, you can fine tune the power delivery and run time of the engine. A larger venturi will increase top end power, while at the same time reducing run time. A smaller venturi will improve bottom end throttle response, and improve run time.