A throttle valve opening control device includes a drum which is connected
to a throttle grip by a wire, an input shaft which is integrally mounted
in the drum, an output shaft which is connected to the input shaft by a
power transmission device, a link mechanism which connects the output
shaft to a forward valve shaft and a rear valve shaft, and a drive motor
which drives the power transmission device to provide the relative
rotation of the output shaft with respect to the input shaft. The input
shaft and the output shaft have respective axes arranged along a straight
line. The drive motor is arranged parallel to the output shaft, and the
output shaft and the drive motor are juxtaposed vertically and
substantially along a centerline of an intake passage. The throttle valve
opening control device is relatively compact to reduce engine size.