A combustion chamber system, for use in connection with internal combustion powered fastener-driving tools, comprises a dual-level combustion chamber comprising first and second pre-combustion and main combustion chambers, and a dual-level rotary valve mechanism which is operatively connected to the dual-level combustion chamber so as to control the intake of the air/fuel mixture as well as the scavenging or purging of the exhaust gases or combustion products. The first and second pre-combustion and main combustion chambers are also separated by a plate having a plurality of apertures defined therein whereby multiple jets or flame fronts are defined throughout the combustion chamber system.