Abstract: A pump body which comprises three chambers, a first chamber communicating with the outer atmosphere and the air intake or the exhaust pipe, a second chamber communicating with the precompression casing of the engine, said second chamber being separated from the first chamber by a movable partition integral with a support held by a spring with a piston resting on the support and sliding axially in a channel when the movable partition displaces the support, and a third chamber having an opening communicating with the fuel intake line and an opening connected by a tube to a fuel injector opening into the drive cylinder in a substantially radial direction so as to spray the fuel over the flushing or sweeping currents.