[The
construction and operation
dealt with the most
common type of carburettor, the side draft Zenith. However, the carburettor is
only of use if it gets a supply of Petrol. This article was found in an undated
“Practical Home Handyman”, found by Richard Butterworth for 30p at the St Mellion Cherry Feast White Elephant stall! ]
In nearly
all modern motor vehicles the petrol tank is situated well below the level of
the carburettor. Consequently, as the petrol will not flow to the float chamber
some means must be provided to deliver fuel to the carburettor. In the past, it
was sometimes the practice to locate the tank under the scuttle and fed the
carburettor by gravity. But now it has become the practice to position the tank
at the rear of the car. This very often involves lifting the petrol a matter of
2 ft. to the carburettor float chamber. Suction pumps which utilized engine
induction pressure for the operation of a vacuum for the delivery of fuel from
tank to carburettor were once very popular, but these are now obsolete having
been replaced by pumps with positive drives. These diaphragm type pumps have now
become standardized fitting on all vehicles. To how these pumps work we will
describe the action of a typical fully mechanical pump.
The pump is
usually bolted to the crankcase of the engine and operated directly by an
eccentric on the camshaft, or by push rod. As the camshaft rotates, the
eccentric lifts the rocker-arm which pulls the connecting rod, together with the
diaphragm, downward against the pressure of the return spring, thus creating a
vacuum in the pump chamber. Fuel from the tank is then sucked through the inlet
connection, into the sediment chamber, through the gauze filter and inlet valve
into the pump chamber. On the return stroke the pressure of the return spring
pushes the diaphragm upward forcing fuel from the pump chamber through the
outlet or delivery valve and outlet connection to the carburettor.
When the
correct level in the carburettor float chamber is reached, the needle valve will
close, thus creating a back pressure in the pump chamber. This pressure will
hold the diaphragm downward against the return spring and it will remain in this
position until the carburettor requires more fuel and the float chamber needle
valve opens.
When the
fuel pump is subjected to a back pressure the diaphragm connecting rod forces
the interior, pivoted portion or lever of the two-piece rocker arm to the bottom
of its stroke. As the outer part of the rocker arm, which is in direct contact
with the eccentric cam, is also secured to the same pivot centre as the lever,
the rocker arm will cease to operate the lever until the diaphragm is returned
to its initial position. The small spring at the rocker arm shoulder is
intended to keep the rocker arm in constant contact with the eccentric, to
eliminate noise.
The gauze
filter should be regularly cleaned. It can be taken out when the cover of the
sediment chamber is removed. Make certain that the cork gasket is properly
seated and that the fibre washer is under the head of the screw when
re-assembling, so that there is no air leak into the sediment chamber. Any
deposit in the sediment chamber can usually be drained off by removing the drain
plug.
Seepage of
fuel at the edge of the diaphragm can generally be cured by tightening up the
body screws. A continual fuel leakage from the drain hole in the body casting
usually indicates a punctured diaphragm. Should this happen it is necessary to
change the diaphragm. This can be achieved by carefully separating the upper and
lower castings once the pump has been removed from the engine block. Simply
remove the body screws and separate the two halves, taking care when refitting
to tighten them sequentially but not excessively.
