Performance of six stroke single power combustion engine affected by venturi diameter of carburetor is presented. This study is to begin on development of six-stroke single power of combustion engine (CE). Developing of this six stroke CE is based on addition of duration on both mass- and thermal diffusion, and on mixing time before the combustion. Evaluation of the six-stroke CE performance is done via enlarge venturi diameter of carburetor. Selected venturi diameters are 18 (mm) and 20 (mm). Fuel of the CE is gasoline-based using research octane number (RON) of 92. Interval of each ascending rotation speed is 400 (rpm), and observation was done using a constant throttle aperture 30 (%). The observed performances of the 125 (cc) six-stroke CE is then compared using a convensional four-stroke 125 (cc), as well. It informs that diameter 20 (mm) can increase average of about 21(%) on torsion, 21 (%) on power, 16 (%) on specific fuel consumption ( SFC ), and 23 (%) on thermal efficiency to that of the diameter 18 (mm). If compared to four-stroke CE, the six-stroke CE has lower in SFC and thermal efficiency, however, it has higher in average value of torsion, power, and engine speed, respectively about 15 (%).