An important part of the engine starting procedure is the priming technique involved. Of course, the pilot’s operating handbook will specify the steps in starting a specific model engine. However, some of the pilot handbooks may not explain why certain procedures are used in the starting process.
Priming can be best accomplished with an engine priming system, as opposed to use of the throttle. The primer pumps extra fuel directly into the cylinder intake port or induction system. Some float-type and pressure carburetors also provide a supplemental source of priming. Lycoming engines of more than 118 HP have a throttle pump which can be used for priming under moderate ambient temperature conditions while turning the engine with the starter.
Pilots should, however, be advised that excessive throttle priming can cause flooding of the carburetor and airbox, and result in a fire in the induction system or on the outside where the fuel drains overboard. If the operator floods the engine by pumping the throttle and has a fire, it is possible to handle such a fire in the early stages by continuing to turn the engine with the starter, thereby sucking the fire back into the engine. Furthermore, if there is any fire on the outside of the engine, if the engine starts there is a good chance it will blow out the external fire.
Flooding of the engine without a fire, the operator should open the throttle full and close the mixture; (see Operator’s Handbook on mixture) and turn the engine over several times with the starter to clear it; then begin again with a normal start routine.
Most Lycoming fuel injected engines are simply primed by turning the fuel boost pump on, opening the mixture briefly to full rich, and cracking the throttle. Any pumping of the throttle is ineffective until the engine begins to fire.
FUEL CONTAMINATION—Water (says the FAA) is the principal contamination of aviation fuel. For a safe flight, carefully drain fuel sumps at each preflight.