Abstract: In a power unit for a saddle-type vehicle a locking pin on a shift drum side is fixed to a shift spindle and a detection shaft of a shift position sensor is interlocked and connected with the shift drum, the friction surface between the shift arm and the locking pin is reduced with suppression of the lowering of the rigidity of the shift arm to enable smooth gear shifting. An engagement hole in a shift arm allows insertion and engagement of a locking pin and has an elongated hole shape extending along one diameter line of a shift spindle. An insertion hole in the shift arm has an arc shape centered at the center axis line of the shift spindle to allow insertion of an interlocking shaft coaxially connected to the shift drum to be incapable of rotation relative to it.

Abstract: In a power unit for a saddle-type vehicle a locking pin on a shift drum side is fixed to a shift spindle and a detection shaft of a shift position sensor is interlocked and connected with the shift drum, the friction surface between the shift arm and the locking pin is reduced with suppression of the lowering of the rigidity of the shift arm to enable smooth gear shifting. An engagement hole in a shift arm allows insertion and engagement of a locking pin and has an elongated hole shape extending along one diameter line of a shift spindle. An insertion hole in the shift arm has an arc shape centered at the center axis line of the shift spindle to allow insertion of an interlocking shaft coaxially connected to the shift drum to be incapable of rotation relative to it.

Abstract: To achieve further reduction in size and weight of a cylinder head in a four-cycle engine having a connection surface formed on the cylinder head to connect a head cover set so as to follow along a plane that is inclined so as to be spaced further away from a cylinder block toward a first side wall. A camshaft holder is integrally formed on a cylinder head so as to protrude from a connection surface as viewed from an axial direction of a camshaft. An intake port is disposed in the first side wall such that an upstream end thereof is disposed at the same position as part of the bearing portion in a direction along an axis of a cylinder bore. An exhaust valve has a stem end disposed at a position protruding from the connection surface as viewed in a direction along the axial direction of a camshaft.

Abstract: An exhaust control device for a vehicle engine wherein an exhaust pipe forms an exhaust passage along with an exhaust port is connected to a cylinder head via the exhaust port. A rotary valve is capable of changing the opening area of the exhaust passage. The rotary valve being disposed in approximately the middle of the exhaust passage. The opening area of the exhaust passage is made controllable by the rotary valve while avoiding an increase in the number of component parts. A rotary valve having a turning axis located at a position offset from the center of the exhaust port is housed in a valve housing provided integrally with a cylinder head.

Abstract: A hydraulic pressure warning system for an internal combustion engine including a hydraulic pressure sensor that is prevented from being influenced by the heat of the engine and by the pulsation of an oil pump. A hydraulic pressure sensor detects hydraulic pressure in a lubricating oil passage of the engine and issues a warning based on hydraulic pressure detected by the hydraulic pressure sensor when the hydraulic pressure drops. The hydraulic pressure sensor is provided on a wall surface of a water jacket of a rear cylinder block so as to project therefrom.

Abstract: An internal combustion engine for reducing the amount of bubbles contained in the circulated oil without reducing the ground clearance of the engine. A crankcase is provided therein with an oil suction passage makes an oil pump and an end part of a bottom wall of an oil sump chamber communicate with each other, passes between the oil sump chamber and a hanger boss, and has a lowest part located below the bottom wall of the oil sump chamber.

Abstract: An exhaust control device for a vehicle engine includes an exhaust control valve capable of changing the opening area of an exhaust passage. The exhaust control valve is disposed in exhaust passage forming means so as to be able to change a bent-internal side partial passage sectional area of the passage sectional area of an exhaust passage. Specifically the exhaust control valve is disposed at a bent portion of the exhaust passage that is closest to an exhaust valve port. With this configuration, the exhaust flow rate can be controlled satisfactorily.

Abstract: In a four-cycle engine, the stiffness of a rocker shaft support portion is increased while avoiding an increase in the size of a cylinder head. The rocker shaft support portion that supports a rocker shaft is integrally formed on the cylinder head so as to be disposed between an engine valve and a camshaft as viewed from an axial direction of the camshaft and the rocker shaft. First reinforcement ribs are integrally disposed in a protruding condition on a plane orthogonal to an axis of a rocker shaft on a side face of rocker shaft support portions opposite a side of a camshaft.

Abstract: A shift drive device with a shift arm having one end secured to a shift spindle with a guide hole for receiving a pin, provide to project therefrom, inserted therethrough and engaged therewith. A turn-restriction hole receives a restriction pin provided on a transmission case. A space that is needed for turning the shift arm is reduced. A turn-restriction hole is provided in a shift arm so as to be disposed on a straight line connecting an axial line of a shift spindle with the center of a guide hole.

Abstract: An internal combustion engine in which it can be made difficult for bubbles to mix into oil in a flowing-down process of the oil. In a generator chamber, a shielding rib is provided which is disposed between a generator and an oil introduction port in side view and which extends skewly upwardly from the oil introduction port in a manner so as to cover one end side of the oil introduction port. A guide rib extends from a side wall of the generator chamber in the manner of covering the other end side of the oil introduction port.

Abstract: An internal combustion engine including a valve system and a breather system. The valve system includes a camshaft having intake cams and exhaust cams. The intake cams open and close intake valves, and the exhaust cams open and close exhaust valves. The breather system includes an upstream-side breather chamber and a downstream-side breather chamber. Blow-by gas flows into the upstream-side breather chamber through a chain chamber. The down-stream side breather chamber is provided in the camshaft, and also formed of a through hole constituted of a plurality of hole portions. The hole portions have different sizes and are aligned in the axial direction. The through hole opens to the chain chamber at a first shaft end portion of the camshaft, and also opens to the down-stream side breather chamber at a second shaft end portion of the camshaft. The down-stream side breather chamber is formed of a cam holder.

Abstract: A hydraulic pressure warning system for an internal combustion engine including a hydraulic pressure sensor that is prevented from being influenced by the heat of the engine and by the pulsation of an oil pump. A hydraulic pressure sensor detects hydraulic pressure in a lubricating oil passage of the engine and issues a warning based on hydraulic pressure detected by the hydraulic pressure sensor when the hydraulic pressure drops. The hydraulic pressure sensor is provided on a wall surface of a water jacket of a rear cylinder block so as to project therefrom.

Abstract: An internal combustion engine for reducing the amount of bubbles contained in the circulated oil without reducing the ground clearance of the engine. A crankcase is provided therein with an oil suction passage makes an oil pump and an end part of a bottom wall of an oil sump chamber communicate with each other, passes between the oil sump chamber and a hanger boss, and has a lowest part located below the bottom wall of the oil sump chamber.

Abstract: An internal combustion engine in which it can be made difficult for bubbles to mix into oil in a flowing-down process of the oil. In a generator chamber, a shielding rib is provided which is disposed between a generator and an oil introduction port in side view and which extends skewly upwardly from the oil introduction port in a manner so as to cover one end side of the oil introduction port. A guide rib extends from a side wall of the generator chamber in the manner of covering the other end side of the oil introduction port.

Abstract: A shift drive device with a shift arm having one end secured to a shift spindle with a guide hole for receiving a pin, provide to project therefrom, inserted therethrough and engaged therewith. A turn-restriction hole receives a restriction pin provided on a transmission case. A space that is needed for turning the shift arm is reduced. A turn-restriction hole is provided in a shift arm so as to be disposed on a straight line connecting an axial line of a shift spindle with the center of a guide hole.

Abstract: In a four-cycle engine, the stiffness of a rocker shaft support portion is increased while avoiding an increase in the size of a cylinder head. The rocker shaft support portion that supports a rocker shaft is integrally formed on the cylinder head so as to be disposed between an engine valve and a camshaft as viewed from an axial direction of the camshaft and the rocker shaft. First reinforcement ribs are integrally disposed in a protruding condition on a plane orthogonal to an axis of a rocker shaft on a side face of rocker shaft support portions opposite a side of a camshaft.

Abstract: To achieve further reduction in size and weight of a cylinder head in a four-cycle engine having a connection surface formed on the cylinder head to connect a head cover set so as to follow along a plane that is inclined so as to be spaced further away from a cylinder block toward a first side wall. A camshaft holder is integrally formed on a cylinder head so as to protrude from a connection surface as viewed from an axial direction of a camshaft. An intake port is disposed in the first side wall such that an upstream end thereof is disposed at the same position as part of the bearing portion in a direction along an axis of a cylinder bore. An exhaust valve has a stem end disposed at a position protruding from the connection surface as viewed in a direction along the axial direction of a camshaft.

Abstract: An exhaust control device for a vehicle engine includes an exhaust control valve capable of changing the opening area of an exhaust passage. The exhaust control valve is disposed in exhaust passage forming means so as to be able to change a bent-internal side partial passage sectional area of the passage sectional area of an exhaust passage. Specifically the exhaust control valve is disposed at a bent portion of the exhaust passage that is closest to an exhaust valve port. With this configuration, the exhaust flow rate can be controlled satisfactorily.

Abstract: An exhaust control device for a vehicle engine wherein an exhaust pipe forms an exhaust passage along with an exhaust port is connected to a cylinder head via the exhaust port. A rotary valve is capable of changing the opening area of the exhaust passage. The rotary valve being disposed in approximately the middle of the exhaust passage. The opening area of the exhaust passage is made controllable by the rotary valve while avoiding an increase in the number of component parts. A rotary valve having a turning axis located at a position offset from the center of the exhaust port is housed in a valve housing provided integrally with a cylinder head.

Abstract: An internal combustion engine including a valve system and a breather system. The valve system includes a camshaft having intake cams and exhaust cams. The intake cams open and close intake valves, and the exhaust cams open and close exhaust valves. The breather system includes an upstream-side breather chamber and a downstream-side breather chamber. Blow-by gas flows into the upstream-side breather chamber through a chain chamber. The down-stream side breather chamber is provided in the camshaft, and also formed of a through hole constituted of a plurality of hole portions. The hole portions have different sizes and are aligned in the axial direction. The through bole opens to the chain chamber at a first shaft end portion of the camshaft, and also opens to the down-stream side breather chamber at a second shaft end portion of the camshaft. The down-stream side breather chamber is formed of a cam holder.