Primer

A small manual or electric pump in the cockpit that injects a measured amount of fuel directly into the engine cylinders to aid cold engine starting. The pilot pumps the primer several times before engaging the starter to provide a combustible fuel-air mixture in the cylinders. After starting, the primer must be locked in the closed position to prevent it from acting as an unmetered fuel leak, which can cause the engine to run rough or create a fire hazard. The number of primer strokes needed depends on engine temperature and the specific POH procedure.