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 TM 55-1905-219-14-8
4-7.11.
PRIMING PUMP AND GEAR HOUSING-MAINTENANCE INSTRUCTIONS.
a. General.
(1) The priming pump is of the positive displacement type, and is mounted on the gear housing
beneath the carburetor. The gear on the crankshaft drives an idler gear, which in turn transmits power to the
gear on the primer pump shaft.
(2) The intake tube to the primer pump is connected to the intake side of the fire pump. A pressure
tube is connected from the impeller housing to the diaphragm side of the primer pump.
(3) When the engine is started, the primer pump evacuates the air from the fire pump impeller
housing and the suction hose. Because of this suction, atmospheric pressure forces water up through the
suction hose and into the impeller housing. As soon as the water enters the center of the rotating impeller, the
fire pump begins to build up pressure. This pressure is carried through the pressure tube from the impeller
housing to the diaphragm on the outboard end of the primer pump. The diaphragm is in contact with the end of
the primer pump drive shaft, and when the pressure builds up, it forces the primer pump drive shaft against the
clutch plate, which separates the clutch face from the primer pump drive gear, disengaging the primer pump.
Therefore, the priming pump is in operation only until the fire pump has been primed.
(4) The gear housing and primer pump are lubricated from the oil that accumulates in the bottom of
the engine crankcase. There are two spring-loaded valves at the bottom of the crankcase which allow the oil to
bleed out the crankcase into rubber lines to the primer pump and gear housing.
b. Description.
(1) The gear housing is mounted between the impeller housing and the power head. It contains a
crankshaft-mounted pinion gear, an idler gear, and the primer drive gear, which are the means of transmitting
power from the crankshaft to the primer pump. As the crankshaft rotates, the pinion gear drives the idler gear,
which in turn drives the primer pump driver gear. A friction clutch arrangement transmits power from the
primer pump drive gear to the primer pump impeller.
(2) The primer pump consists of two end plates, a housing, impeller, and vanes. The impeller is
mounted off-center within the housing. Located axially in slots along the impeller surfaces are the four vanes.
These vanes are free to move, and, as the impeller rotates, they move outward due to centrifugal force, until
they contact the inside of the housing. They thus trap air drawn from the impeller housing and suction hose
between the housing, the impeller, and the vanes. Air trapped in this manner is forced to the primer pump
outlet.
4-278
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