A single cylinder, 4-stroke spark ignition engine type Prodit; fueled with supplementary Natural gas to liquefied petroleum gas (LPG) was used in this paper to investigate the emitted CO pollutants. The effect of equivalence ratio, spark ignition timing, engine speed, and the added NG volumetric ratio on CO emissions were tested experimentally. The study outcomes revealed that CO levels depend mainly on the equivalence ratio, as the maximum value of CO concentrations existed at the very rich equivalence ratios and it low at lean side. Retarding spark timing increased the CO concentrations by a significant percentage. The CO levels became higher at high speeds and reduced at medium speeds. Increasing natural gas volumetric ratio in the mixture caused a reduction in CO levels.