In this study, the mass of THC is reduced almost 40 percent with spark timing ATDC 7.8CA during 15 seconds from engine start in phase 1 LA4 mode, comparison with TDC 2.8CA (Figure 1, Table 3). One of the reason of HC reduction in vehicle test is reduction of raw THC concentration before CCC (Closed Coupled Catalyst) which is 36% lower level (Figure 3, Table 3). The other reason is that the LOT (Light Off Time) of catalyst is shortened from 34 seconds to below 20 seconds (Figure 7, Table 3). As the spark timing is retarded with same intake air quantity and same RPM, BMEP is reduced (Equation (3), Figure 9). Therefore in order to operate in an idle RPM in vehicle, the mass of intake air should be increased (Figure 5). So a catalyst is heated in shorter period. In order to find out the reason of THC reduction with spark timing (Figure 9), THC concentration is measured at an exhaust port and manifold in single cylinder engine at the condition of AFR 15.0 (Figure 11, Table 4) and AFR 13.0 (Figure 12, Table 5). And flame propagation was visualized as well (Figure 13, Figure 14, Figure 15). Spark timing is closely correlated with the duration of combustion (Figure 13, Figure 14). And the combustion duration is correlated with the evaporation of fuel wetted on wall and THC emission. As a result THC concentration is inversely proportional to the lasting time of flame (Figure 11, Figure 12, Figure 13, Figure 14). The applicability of the spark timing retard method previously mentioned with commercial vehicle with normal gasoline engine (Table 1) is evaluated by RPM fluctuation during the operation period with spark timing ATDC 7.8CA in cold idle condition (Figure 4, Table 3) and air conditioner was on or off (Figure 8, Table 2). Combustion stability of engine (1) such as IMEP COV (Equation (1)) or IMEP LNV (Equation (2)) at coolant temperature 30 °C was measured. As a result, there is not much different between BTDC 2.8CA and ATDC 7.8CA spark timing operation because engine stability is not only correlated with spark timing but also correlated with intake MAP (Manifold Absolute Pressure) of an engine (Figure 10). As the air conditioner is on, the load for air conditioner operation is covered by a little spark timing advance (Figure 8, Table 2).