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ENGINE FUEL AND EXHAUST SYSTEMS
FUEL AND VACUUM PUMP
3-19
section of the pump. See figure 3-10. Fuel from
the gasoline tank first enters the bowl then
flows upward through the screen into the fuel
pump. The bowl provides a settling chamber
for water and the dirt which cannot pass the
screen. The bowl and screen should be cleaned
periodically
(par.
3-9).
b. Operation of Fuel Section of Pump
The function of the fuel section of the pump
is to draw gasoline from the tank and supply
it to the carburetor in sufficient quantity to
meet engine requirements at all speeds and
loads.
The pump rocker arm (G) is actuated by
an eccentric on engine
camshaft.
The arm is
held in contact with the eccentric by the rocker
arm spring (H). Movement of rocker arm is
transmitted to the fuel link (E) which pulls
the fuel diaphragm
(I)
upward from the fuel
chamber
(J)
against pressure of the dia-
phragm spring (N). See figure 3-25.
Vacuum created in the fuel chamber by up-
ward movement of diaphragm holds the outlet
valve (M) closed and causes fuel to flow from
the gasoline tank, through the inlet valve (not
shown), into the fuel chamber.
The return stroke (low point of cam) re-
leases the compressed diaphragm spring which
then exerts pressure on the diaphragm and the
fuel in the
chamber.
This pressure closes the
inlet valve and forces fuel out through the out-
let valve to the carburetor in an amount gov-
erned by the pressure in the
pump-to-carbu-
retor
line.
The fuel link is hinged to the rocker arm so
that the link and the connected fuel diaphragm
can be moved up
,
but not down,
.by
the rocker
arm. The link and the diaphragm are moved
downward only by the diaphragm spring. The
pump, therefore, delivers fuel to the carbure-
tor only when the fuel pressure in the outlet
line is less than the pressure maintained by
the diaphragm spring. This condition arises
when the carburetor float needle valve is not
seated and the fuel passage from the pump
into the carburetor
float
chamber is
open.
When t
he needle va
lve
is closed and held in
place by the pressure of
the
fu
el
on the
float,
the pump builds up pressure in fuel chamber
until it overcomes the pressure of the dia-
phragm spring. This pressure results in almost
complete stoppage of diaphragm movement
until more fuel is needed. Normal diaphragm
stroke is aproximately
lIB/'.
The air dome used on Series 70 pumps pro-
vides a pocket
In
which fuel under pressure
can compress a certain volume of air. When
the pressure is relieved (pump on su
ction
stroke) the pocket of compressed air pushes
the fuel on to its destination. The air dome
minimizes flow variations experienced with a
two-cycle pump stroke and increases the pump
output.
c.
Operation of Vacuum Section of Pump
The function of the vacuum
section
of the
pump is to act as
a
booster to the intake mani-
fold vacuum, thereby providing uniform opera-
tion of the windshield wiper at all engine
speeds and loads.
The pump rocker arm (G) is actuated by
an eccentric on engine camshaft. The
a
rm is
held in contact with the eccentric
by
the
r
ocker
arm spring (H)
. Movement
of rocker arm is
transmitted to
the
vacuum link
(0)
which
pushes vacuum diaphragm (D)
upward
into
air chamber
(Q)
against pressure of the dia-
phragm spring (C)
.
See figure
3-25.
Pressure
created
in the air chamber by up-
ward movement of diaphragm holds the inlet
valve (not shown) closed and expels air
through the outlet valve (A) into the engine
manifold.
The return stroke
(low point of cam)
releases the compressed diaphragm spring
which then pushes the diaphragm
down,
creat-
ing a vacuum in air chamber . This vacuum
closes the outlet valve and draws air through
the inlet valve
from
the windshield wiper.
The diaphragm operates only when engine
vacuum is insufficient for windshield wiper
action.
When manifold vacuum is greater than
that created by the pump, the stronger mani-
fold vacuum pulls the diaphragm into the air
the vacuum link out of engagement with the
rocker arm. The windshield wiper then
oper-
ates on manifold vacuum without assistance
from the pump. When intake manifold vacuum
is low, as on acceleration or at high speed,
the vacuum created
by t
he
pum p will
assur e
ad
equat e operation of the wiper.
3-17 FUEL PUMP INSPECTION AND
TEST
If
the fuel pump is suspected of delivering
an improper amount of fuel to the carburetor,
it should be inspected and tested on the engine,
as follows
:
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