A manually operable priming pump has a housing formed with an inlet port adapted to be connected to a supply of fuel and an outlet port adapted to be connected to a fuel-delivery system, a body in the housing between the ports and formed with a throughgoing main passage that is completely clear of obstructions. The body is shiftable between an operating position in which the main passage is aligned with the inlet and outlet ports and permits free flow therebetween and a pumping position, and a piston in the body can be manually reciprocated in the pumping position to draw fluid in through the inlet port and expel it from the outlet port.