Control emission pollution associated with oil combustion is a major concern of researchers worldwide. A blend of HHO has been introduced to the combustion elements to reduce emission and drive the combustion reaction toward stoichiometric condition. HHO fuel production unit based on an electrolysis process has been designed and built with an ability to alter Anode-Cathode plate's inter-distances and integrated to Honda G 200 (197 cc single cylinder engine). The gap between the plates was adjusted to 3, 5, 7 and 10 mm. Tests reveal that mixing HHO, air, and gasoline cause an enhancement in engine performance and emissions. The emission tests have been done with varying the engine speed and preserve the electrolyte concentration and temperature. The results show that enhancement in combustion characteristics is strongly affected by the gap between cell plates. The maximum produced power and minimum fuel consumption were associated with the case of 10 mm cathode-anode plates distance where hydrocarbons (HCs) and carbon monoxide emissions have been reduced to about 40% at different operating speeds. Whereas, 5 mm gap case has the highest impact on emission reduction. (C) 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.