An engine setting system comprising a vehicle, a setting means (90, 95) arranged outside the vehicle, and a control device mounted on the vehicle which controls driving of a vehicle engine, wherein the setting means (90, 95) and the control device can be communicably connected to each other so that an engine control variable stored in the control device is changeable by the setting means (90, 95), wherein: the vehicle is a motorcycle without a battery; wherein the engine setting system is configured such that an input-output cable can be connected to a coupler of the control device and to an external interface box so that the control device is connected to the setting means (90, 95) by way of the external interface box; and wherein a battery is connected to the external interface box so that electric power can be supplied to the control device; characterised in that the control device comprises: a bypass passage opening adjusting means which adjusts opening of a bypass passage which bypasses a throttle valve arranged in an intake passage connected to the engine based on a rotational speed of the engine; and an opening storing means which stores target opening of the bypass passage opening adjusting means corresponding to the rotational speed of the engine; in that the setting means (90, 95) is capable of changing the target opening stored in the opening storing means; and in that the target opening can be set by a user of the vehicle via the external interface box between an upper limit value and a lower limit value; wherein the target opening is set with respect to at least two predetermined engine rotational speeds, including a first rotational speed and a second rotational speed, which are set in advance; wherein the target opening in a rotational speed region between the first rotational speed and the second rotational speed is set by linear interpolation using the target openings corresponding to the first rotational speed and the second rotational speed; wherein, in a rotational speed region where the engine rotational speed is smaller than the first rotational speed, the target opening is set to correspond to the target opening at the first rotational speed, and in a rotational speed region where the engine rotational speed is larger than the second rotational speed, the target opening is set to correspond to the target opening at the second rotational speed; and wherein the first rotational speed is set to be equal to or more than an idling rotational speed, and the second rotational speed is set to be larger than the first rotational speed, and a separation between the upper and lower limit values of the target opening which is changeable at the first rotational speed is set to be smaller than a separation between the upper and lower limit values of the target opening which is changeable at the second rotational speed. the vehicle is a motorcycle without a battery; wherein the engine setting system is configured such that an input-output cable can be connected to a coupler of the control device and to an external interface box so that the control device is connected to the setting means (90, 95) by way of the external interface box; and wherein a battery is connected to the external interface box so that electric power can be supplied to the control device; characterised in that the control device comprises: a bypass passage opening adjusting means which adjusts opening of a bypass passage which bypasses a throttle valve arranged in an intake passage connected to the engine based on a rotational speed of the engine; and an opening storing means which stores target opening of the bypass passage opening adjusting means corresponding to the rotational speed of the engine; in that the setting means (90, 95) is capable of changing the target opening stored in the opening storing means; and in that the target opening can be set by a user of the vehicle via the external interface box between an upper limit value and a lower limit value; wherein the target opening is set with respect to at least two predetermined engine rotational speeds, including a first rotational speed and a second rotational speed, which are set in advance; wherein the target opening in a rotational speed region between the first rotational speed and the second rotational speed is set by linear interpolation using the target openings corresponding to the first rotational speed and the second rotational speed; wherein, in a rotational speed region where the engine rotational speed is smaller than the first rotational speed, the target opening is set to correspond to the target opening at the first rotational speed, and in a rotational speed region where the engine rotational speed is larger than the second rotational speed, the target opening is set to correspond to the target opening at the second rotational speed; and wherein the first rotational speed is set to be equal to or more than an idling rotational speed, and the second rotational speed is set to be larger than the first rotational speed, and a separation between the upper and lower limit values of the target opening which is changeable at the first rotational speed is set to be smaller than a separation between the upper and lower limit values of the target opening which is changeable at the second rotational speed.