PURPOSE: To improve riding comfort as well as to prevent the occurrence of abnormal combustion of an engine by closing an air vent passage with an electromagnetic valve turned off when vehicle speeds exceed a specified one, allowing atmospheric air to come in to a blast chamber so that the quantity of fuel supply is increased, and thereby controlling vehicle speeds to be lowered.
CONSTITUTION: When it is detected by the detected signal from a vehicle speed sensor that vehicle speeds come up to the controlled vehicle speed value, an air vent passage 30 is closed with a valve body 36 moved by the energizing force of a spring 38, and a ball vent valve passage 32 is then opened, so that atmospheric air is thereby introduced into a float chamber 4 with the float chamber 4 opened atmosphere. In this case, pressure within the float chamber 4 is increased with atmospheric air introduced, fuel is fed into an engine out of a main nozzle 24 and a needle jet 14, the quantity of fuel supply to the engine is increased, and vehicle speeds are controlled to be lowered with an air-fuel ration made rich while an engine output rate is being made small.
YAMAMOTO HIROSHI
YOKOO MASAO
