A fastener-driving tool comprising: a first combustion cylinder partially defining a first combustion chamber; a fuel assembly configured to direct a fuel/air mixture to the first combustion chamber; a second combustion cylinder attached to the first combustion cylinder and partially defining a second combustion chamber; and a combustion chamber sealing assembly comprising a combustion chamber shaft, a first seat disc fixedly attached to the combustion chamber shaft, a second seat disc slidably mounted to the combustion chamber shaft, and a second seat disc biasing element that biases the second seat disc to a biased position, wherein when the combustion chamber sealing assembly is in an unsealed configuration, the first seat disc is disengaged from a first valve seat and the second seat disc is disengaged from a second valve seat such that the fuel assembly, the first combustion chamber, and the second combustion chamber are in fluid communication, wherein when the combustion chamber sealing assembly is in a sealed configuration, the first seat disc sealingly engages the first valve seat and the second seat disc sealingly engages the second valve seat such that the fuel assembly, the first combustion chamber, and the second combustion chamber are not in fluid communication, and wherein when the combustion chamber sealing assembly is in a partially sealed configuration, the first seat disc sealingly engages the first valve seat and the second seat disc is displaced from the biased position and disengaged from a second valve seat such that the fuel assembly and the first combustion chamber are not in fluid communication and the first combustion chamber and the second combustion chamber are in fluid communication, characterized in that a divider is positioned within the second combustion chamber to control the flow of the air/fuel mixture for even distribution from a lower portion of the second combustion chamber to an upper portion of the second combustion chamber. a first combustion cylinder partially defining a first combustion chamber; a fuel assembly configured to direct a fuel/air mixture to the first combustion chamber; a second combustion cylinder attached to the first combustion cylinder and partially defining a second combustion chamber; and a combustion chamber sealing assembly comprising a combustion chamber shaft, a first seat disc fixedly attached to the combustion chamber shaft, a second seat disc slidably mounted to the combustion chamber shaft, and a second seat disc biasing element that biases the second seat disc to a biased position, wherein when the combustion chamber sealing assembly is in an unsealed configuration, the first seat disc is disengaged from a first valve seat and the second seat disc is disengaged from a second valve seat such that the fuel assembly, the first combustion chamber, and the second combustion chamber are in fluid communication, wherein when the combustion chamber sealing assembly is in a sealed configuration, the first seat disc sealingly engages the first valve seat and the second seat disc sealingly engages the second valve seat such that the fuel assembly, the first combustion chamber, and the second combustion chamber are not in fluid communication, and wherein when the combustion chamber sealing assembly is in a partially sealed configuration, the first seat disc sealingly engages the first valve seat and the second seat disc is displaced from the biased position and disengaged from a second valve seat such that the fuel assembly and the first combustion chamber are not in fluid communication and the first combustion chamber and the second combustion chamber are in fluid communication, characterized in that a divider is positioned within the second combustion chamber to control the flow of the air/fuel mixture for even distribution from a lower portion of the second combustion chamber to an upper portion of the second combustion chamber.