A combustion-powered fastener-driving tool (10) includes a combustion-powered power source (14) including a cylinder head (42) and a combustion chamber (18) defined by the cylinder head, a valve sleeve (36) and an upper surface of a reciprocating piston (22), the valve sleeve reciprocable relative to the cylinder head between a rest position and a pre- firing position. The valve sleeve (36) has a range of positions between a first sealing position in which the combustion chamber (18) is closed, and said pre-firing position in which the valve sleeve is prevented from further movement. A lockout device (60, 90, 100, 110) is associated with the power source and has an actuated position configured for preventing the reciprocation of the valve sleeve beyond the first sealed position to open the combustion chamber, but permitting movement of the valve sleeve from the first sealed position to the pre-firing position until the piston returns to a piston pre-firing position post combustion.