A gas-powered tool motor includes a combustion chamber with an intake valve at one end, an exhaust valve at another end, and a control plate or control valve between two portions of the combustion chamber. A piston or other positive displacement device is in communication with the combustion chamber. The intake and exhaust valves have closure members that are movable along a common axis in tandem between collective open positions for recharging the combustion chamber with the fuel and air mixture and collective closed positions for detonating the fuel and air mixture in the combustion chamber and displacing the positive displacement device. The control plate or control valve supports limited air flows from a first portion of the combustion chamber to a second portion of the combustion chamber even in the closed position of the control valve for supporting two-stage combustion.