A four stroke engine has a camshaft driven by a crankshaft. A decompression member is disposed on the camshaft. The decompression member is movable between a decompression position and a non-decompression position. A portion of the decompression member generally places the valve at an open position when the decompression member is placed at the decompression position and releases the valve from the open position when the decompression member is placed at the non-decompression position. A regulating member is disposed on the camshaft. The regulating member regulates the decompression member to prevent movement of decompression member from the decompression position when the regulating member is placed at a first position. The regulating member releases the decompression member from the decompression position when the regulating member is placed at a second position. A bias member is arranged to urge the regulating member toward the first position. The regulating member is movable toward the second position against the urging force of the bias member when a rotational speed of the camshaft exceeds a predefined speed.