Abstract: A novel spark plug adapter unit for spark plugs of an internal combustion engine having a main combustion chamber defined by a cylinder bore and its associated piston and a valved intake passage for introducing a lean fuel-air mixture in the combustion chamber. The spark plug adapter mounted in the engine in the threaded socket for the conventional spark plug has a mini-combustion or spark ignition chamber about the spark gap and a valved branch passage to the ignition chamber connected to a first fuel supply, such as a raw fuel source or a rich fuel-air mixture source, to achieve ignition thereof in the ignition chamber causing a flame discharge which ensures ignition of another fuel mixture, such as a lean mixture, in the main combustion chamber.