PURPOSE: To prevent generation of engine stall by a method wherein the establishments of each conditions of the meshing of a change gear and the engagement of a clutch are detected when the engine is being transferred form an idling operation to an accelerating operation in order to increase auxiliary air by a predetermined amount and, thereafter, to decrease it gradually.
CONSTITUTION: When the change gear is set to a first speed position and a throttle valve 5 is opened, the revolution number Ne of the engine is increased. During a period wherein the throttle valve 5 is opened and the clutch is connected, the amount of auxiliary air is held in the amount same as the amount supplied in a loop immediately before this period. Subsequently, when the clutch is connected, the amount of the auxiliary air is increased by a predetermined amount and is supplied to the engine 1 simultaneously with the connection of the clutch. Danger in which the engine stall is generated is decreased in accordance with the increase of the revolution number Ne of the engine after the connection of the clutch and the amount of the auxiliary air is decreased by a a constant volume at every inputs of the signals of a TDC and is supplied to the engine 1.