PURPOSE: To have a high output power both at low and high speed by inserting the tail part of the exhaust pipe in the expansion chamber of maffler, putting an external cylinder rotatably in the tail part of exhaust pipe, and allowing this external cylinder to rotate in accordance with the revolving speed of engine, and thereby putting the exhaust pipe in communication with the expansion chamber selectively.

CONSTITUTION: In low speed range of engine, a plurality of horizontal holes 4 provided in that portion of exhaust pipe 1, which is inserted into the expansion chamber 3 of muffler 2, are shut by an external cylinder 5 fitted on the exhaust pipe 1 rotatably. This secures the exhaust pipe 1 with sufficient length as well as provides pulsation effect in the low speed range to lead to high output. In the high speed range, on the other hand, said external cylinder 5 is rotated by a lever 7 working interlocked with revolutions of the engine so that horizontal holes 6 provided in the external cylinder 5 mate with the above-mentioned horizontal holes 4 in the wall of exhaust pipe 1. This allows the exhaust gas, which comes up through the exhaust pipe 1, to flow into the expansion chamber 3 through the horizontal holes 4, 6. Thus the same effect is obtained as shortening of exhaust pipe 1, which can enhance the output through accomplishment of pulsation effect even in high speed range.