In spark-ignition engine pumping loss increases and fuel economy decreases during partial load operation. Methods to reduce this pumping loss by controlling the intake-valve closing timing are currently under study. The authors, also, have confirmed that pumping loss can be reduced by controlling the amount of intake air-fuel mixture through making changes in the in Cake-valve closing timing.However, when pumping loss was reduced by controlling intake-valve closing timing, an improvement in fuel economy equivalent to the reduction in pumping loss was not obtained.In this study, it was found that the major contributing factor to this phenomenon was the deterioration of the combustion, namely, increase in combustion duration and in combustion fluctuation. Therefore, an analysis was conducted on how the various factors which influence the combustion, such as the gas temperature, pressure and residual gas fraction in the cylinder during the compression stroke, change when the intake-valve closing timing is modified. As a result of this analysis, which was carried out through experiments and simulation based on computations, it was found that the principal cause of the combustion deterioration was the drop in cylinder gas temperature and pressure which was traced to a decrease in the effective compression ratio.