Abstract: 168F spark ignition engine was discussed to reduce the exhaust emissions for meeting the EPA emission standard in this paper. The two-phase flow model of the air-fuel mixture inside the intake system was built by FLUENT. The laws between the structure parameters of carburetor and the excess air ratio were calculated under different working conditions of the engine. According to the testing cycles of the emission standard set by EU and EPA, the formation mechanism of the exhaust emissions was discussed, the characteristic of the excess air ratio for the low-emission S.I. engine was derived, and the air-fuel mixture concentration gradually leant with the engine load reducing. The carburetor parameters for the low emissions S.I. engine were obtained based on the simulation results. The sample engine matching the optimized carburetor was tested. The testing results showed that the NOX and HC emissions decreased by 35.4% and 20.3% respectively compared to the original engine. The optimized sample engine can meet the requirements of the emission standard of EPA which is planned to implement in 2012.