when the engine speed is low only a small centrifugal force acts on the balls 36 arranged between the saucer shaped ball retaining plate 34 and the opposing ball retainig plate 35 because the rotation speed of the rotary shaft 26 is correspondingly low therefore the slider 27 does not slide apprecialby therefore the pin 29 lever 24 vertical shaft 23 and the lever 22 do not rotate so that the valve rod 16 connected to the lever 22 through the pin 21 does not slide in consequence the valve 15 remains to close the sub passage 14 in this state the exhaust port system provides an area of exhaust passage constituted solely by the sectional area of the main exhaust passage 13 which suits with the low speed engine operation as the engine is accelerated the speed of the rotary shaft 26 of the governor 25 and hence the centrifugal force acting on the balls 36 between the ball retainer plates 34 and 35 are increased correspondingly in consequence the balls 36 are displaced radially outwardly urging the ball retaining plate 34 to the left as viewed in fig3 against the force of the spring 33 so that the slider 27 integral with the ball retaining plate 34 is moved to the left this movement causes rotation of the pin 24 vertical rotary shaft 23 lever 22 and other parts shown in fig2 and 4 as a result the valve rod 16 connected to the lever 22 through the pin 21 slides in the bore 19 formed in the plug 18 whereby the valve 15 is retracted into the sleeve 17.