Perhaps the best-
known mechanical
type of fuel pump is
the A.C. This is fitted
on a large number of
different makes of
motor vehicle; the in
dividual designs vary
somewhat in detail,
but the principle re
mains the same.
The A.C. petrol
pump is of the dia
phragm type operated by a lever that is given a reciprocating motion of 3/16 in. to
1/4 in. from camshaft or other rotating part of the engine. The pump draws
fuel from the rear tank through a sediment chamber and strainer, which is
a part of the pump unit, and feeds it under pressure to the carburettor in
strict proportion to the requirements of the engine.
Operation of A.C. Fuel Pump. - The operation of the fuel pump is
illustrated in the case of the model fitted to the Austin A40 engine in
Fig. 10.
As the engine camshaft Q, revolves, the eccentric P lifts the pump
rocker arm R, pivoted at T, which draws the pull rod, together with the
diaphragm H, downward against the spring pressure J, thus creating a
vacuum in the pump chamber G. Fuel is drawn from the rear tank
and enters at D into the sediment chamber E through a gauze filter A
and suction valve N into the pump chamber G. On the return stroke the
spring pressure J pushes the diaphragm H upwards, forcing fuel from the
Fig. 9. - The A.C. Petrol Pump.
A, Filter Gauze.
B, Delivery Port.
C, Delivery Valve.
D, Inlet Union.
E, Sediment Chamber.
F, Sediment Drain Plug.
G, Pump Chamber.
H, Diaphragm.
J, Diaphragm Spring.
K, Diaphragm Pull Rod.
L, Priming Lever.
M, Cork Sealing Washer.
N, Suction Valve.
O, Anti-rattle Spring.
P, Camshaft Eccentric.
Q, Camshaft.
R, Rocker Arm.
S, Connecting Link.
T, Rocker-arm Pivot.
U, Priming Lever Cam.
Fig. 10. - The A.C. Fuel Pump.
chamber G through the delivery valve C and opening B to the carburettor.
When the carburettor bowl is full the float will shut the needle valve, thus
preventing any fuel from leaving the pump chamber G. This will hold
the diaphragm H downward against the
spring pressure J, and it will remain in this
position until the carburettor requires a
further supply of fuel and the needle valve
opens. The rocker arm R operates the con
necting link by making contact at S, and this
construction allows an idling movement of
the rocker arm when there is no movement
of the fuel-pump diaphragm.
The spring O keeps the rocker arm R
in constant contact with the eccentric P to
eliminate noise. The hand priming lever is
indicated at L and the sediment chamber
drain plug at F.
Cleaning the Filter (Fig. 11). - The filter
should be examined every 1,000 miles
and cleaned if necessary. Under extreme
conditions of dust-laden atmosphere the
filter should be examined more fre
quently.
Access to the filter is gained by removing
the dome cover after unscrewing the retaining
screw, when the filter gauze may be lifted
off its seating. Remove the drain plug, and
Fig. II. - Partly Dismantled Fuel Pump
of Austin A40 Engine.
A, Outlet Union. B, Drain Plug.
C, Inlet Union. D, Filter Gauze. E,
Cork Washer. F, Cover Securing Screw.