Valve cam mechanism for four-cycle engine

A valve cam mechanism for a four-cycle engine having a combustion chamber, comprising a valve arranged to open and close the combustion chamber to the atmosphere, a camshaft extending generally vertically, the camshaft being arranged for rotation and having a cam lobe to actuate the valve, and a decompression actuator affixed to the camshaft for pivotal movement about a pivot axis extending generally normal to an axis of the camshaft, the actuator including a holder section and a weight section, the holder section being arranged to hold the valve in an open position when the actuator exists in an initial position, the weight section being disposed opposite to the holder section relative to the pivot axis so as to place the holder section in the initial position, the actuator pivoting when the weight section moves by centrifugal force produced by the rotation of the camshaft so as to release the holder section from the initial position, the actuator and the camshaft together defining means for inhibiting the actuator from pivoting beyond a preset range, the cam lobe including a base circle portion and a nose portion, the base circle portion having a diameter which defines the base circle of the cam lobe, the nose portion protruding from the base circle so as to lift the associated valve, the nose portion being cured so as to be harder than the base circle portion, the base circle portion defining a surface projecting toward the actuator, the holder section abutting on the surface when the actuator lies in the initial position, and the surface being finished by a machining process.

An internal combustion engine comprising a cylinder block defining at least one cylinder bore, a piston reciprocating in the cylinder bore, a cylinder head member closing one end of the cylinder bore and defining a combustion chamber with the cylinder bore and the piston, an intake passage having an intake port through which air is introduced into the combustion chamber, an intake valve arranged to open and close the intake port, an exhaust passage having an exhaust port through which exhaust products are discharged from the combustion chamber, an exhaust valve arranged to open and close the exhaust port, at least one camshaft extending generally vertically, the camshaft being arranged for rotation and having cam lobes to actuate the intake valve and the exhaust valve, and a decompression actuator affixed to the camshaft for pivotal movement about a pivot axis extending generally normal to an axis of the camshaft, the actuator including a holder section, a weight section and a stopper section, the holder section being arranged to hold the intake valve or the exhaust valve in an open position when the actuator exists in an initial position, the weight section being disposed opposite to the holder section relative to the pivot axis so as to place the holder section in the initial position, the actuator pivoting when the weight section moves by centrifugal force produced by the rotation of the camshaft so as to release the holder section from the initial position, the camshaft further having a projection extending radially from the camshaft and defining a first surface on which the stopper section abuts so as to inhibit the actuator from pivoting beyond a preset range, at least the cam lobe associated with the intake valve or the exhaust valve held by the holder section including a base circle portion and a nose portion, the base circle portion having a diameter which defines the base circle of the cam lobe, the nose portion protruding from the base circle so as to lift the associated intake valve or the exhaust valve, the nose portion being cured so as to be harder than the base circle portion, the base circle portion defining a second surface projecting toward the actuator, the holder section abutting on the second surface when the actuator lies in the initial position, and the second surface being finished by a machining process.