An internal combustion engine is disclosed which includes first and second cylinder units each including at least one cylinder, a carburetor with first and second induction passages each having therein a throttle valve rotatable in phase with the other, and first and second intake passages connecting the first and second induction passage to the first and second cylinder units, respectively. The first and second intake passages are communicated near their upstream end. A swing valve is provided which is normally placed in a first position interrupting communication between the first and second intake passages and rotatable to a second position interrupting communication between the second induction passage and the second intake passage. Means are provided for rotating the swing valve to the second position at low load conditions.