A forced air-cooled vehicle engine unit comprising: an engine body including a crank case accommodating a crank shaft, a cylinder block connected to the crank case and accommodating a piston in a reciprocally movable manner, and a cylinder head that is cooperative with the cylinder block to define a combustion chamber and defines a part of a main air intake passage connected to the combustion chamber; an air intake pipe connected to the cylinder head and cooperative with the cylinder head to define the main air intake passage; a shroud covering at least a part of the cylinder head; a fan disposed between the shroud and the engine body, and adapted to be driven by rotation of the crank shaft to generate cooling wind for cooling the engine body; a throttle body including two throttle valves (40A, 40B) spaced from each other in an intake air flowing direction in the air intake pipe, and a tubular member that defines a part of the air intake pipe and accommodates the two throttle valves (40A, 40B), the throttle body being disposed outside the shroud; a fuel injection device attached to the cylinder head and having an injection nozzle which injects fuel into the part of the main air intake passage defined in the cylinder head; characterised in that it further comprises an auxiliary air intake passage defining member that defines an auxiliary air intake passage branched from the main air intake passage between the two throttle valves (40A, 40B) to guide intake air into a space defined adjacent to the injection nozzle in the cylinder head at least during idling; and an air passage (Q1, Q2) that is provided around at least a part of the auxiliary air intake passage for communication between an inside and an outside of the shroud. an engine body including a crank case accommodating a crank shaft, a cylinder block connected to the crank case and accommodating a piston in a reciprocally movable manner, and a cylinder head that is cooperative with the cylinder block to define a combustion chamber and defines a part of a main air intake passage connected to the combustion chamber; an air intake pipe connected to the cylinder head and cooperative with the cylinder head to define the main air intake passage; a shroud covering at least a part of the cylinder head; a fan disposed between the shroud and the engine body, and adapted to be driven by rotation of the crank shaft to generate cooling wind for cooling the engine body; a throttle body including two throttle valves (40A, 40B) spaced from each other in an intake air flowing direction in the air intake pipe, and a tubular member that defines a part of the air intake pipe and accommodates the two throttle valves (40A, 40B), the throttle body being disposed outside the shroud; a fuel injection device attached to the cylinder head and having an injection nozzle which injects fuel into the part of the main air intake passage defined in the cylinder head; characterised in that it further comprises an auxiliary air intake passage defining member that defines an auxiliary air intake passage branched from the main air intake passage between the two throttle valves (40A, 40B) to guide intake air into a space defined adjacent to the injection nozzle in the cylinder head at least during idling; and an air passage (Q1, Q2) that is provided around at least a part of the auxiliary air intake passage for communication between an inside and an outside of the shroud.