Disclosed is a self-priming portable powered device such as a tool, comprising a housing within which are defined a priming combustion chamber and a main combustion chamber, within which chambers during operation of the device there is a combustion of a combustible fuel and a combustion-supporting gas; and wherein a combustion event in the priming combustion chamber causes the formation of a mixture of fuel and combustion-supporting gas in the main combustion chamber which is compressed at a pressure above ambient pressure, subsequent combustion of the compressed mixture in the main combustion chamber acting on a work piston which, directly or indirectly, performs the intended task.