Fuel System; Construction And Operation Of Carburetor; Operation Of Fuel Pump - Stihl 009 User Manual

STIHL 009, 010, 011
8.

FUEL SYSTEM

8.1 Construction and
Operation of Carburetor
The all-position diaphragm
carburetor consists of the fuel pump
and the carburetor body. The fuel
pump operates as a completely
separate and independent unit
although it shares a common
housing with the carburetor.
8.1.1

Operation of Fuel Pump

The pressure in the crankcase varies
with each stroke of the piston. The
piston creates a depression in the
crankcase on its upward stroke and
overpressure on its downward stroke.
This is utilized for actuation of the
fuel pump. The chamber in front of
the pump diaphragm (pulse
chamber) is connected to the crank
chamber by a pulse line. The
changes in pressure act directly on
the pump diaphragm and cause it to
move in time with the piston. Control
is effected by means of two flap
valves stamped in the pump
diaphragm.
The depression created by the up-
ward stroke of the piston draws the
pump diaphragm into the diaphragm
chamber. This enlarges the pump
chamber and produces a vacuum.
The inlet valve then opens and the
higher atmospheric pressure forces
fuel from the tank into the pump
chamber and presses the outlet valve
against its seat.
The downward stroke of the piston
changes the relative pressures. An
overpressure is built up in the crank
and pump chambers which presses
the diaphragm against the pump
chamber and exerts pressure on the
fuel. The overpressure forces the
inlet valve to close, while the outlet
valve opens and allows fuel to flow to
the carburetor's needle valve.
8.1.2 Operation of Carburetor
The opening and closing action of
the needle valve and, therefore, the
supply of fuel to the carburetor is
controlled by the metering
diaphragm. The metering diaphragm
is in a position of rest when
atmospheric and diaphragm chamber
pressures are equal (the chamber
above the diaphragm is connected to
atmosphere).
The cone of the inlet needle is held
against its seat by spring pressure.
The metering diaphragm chamber is
filled with fuel when the engine is
running. A depression is created in
the choke tube (venturi) during the
induction stroke. Fuel is drawn into
the choke tube through the jet bores
between the choke tube and dia-
phragm chamber. This is turn pro-
duces a depression in the diaphragm
chamber and atmospheric pressure
presses the metering diaphragm to-
ward the carburetor body. The force
generated by the pressure difference
x diaphragm area acts on the inlet
control lever via the perforated disc
on the diaphragm, overcomes the
spring force and lifts the inlet needle
off its seat. This allows fresh fuel to
flow from the pump chamber into the
diaphragm chamber. The needle
valve closes again as soon as atmo-
spheric pressure is reached in the
metering chamber. Under normal
operating conditions the needle valve
does not open and close constantly.
The metering diaphragm actually
settles down to a mean level, depen-
ding on engine speed, and the
needle valve remains open relative to
the diaphragm's position.
The quantity of fuel drawn into the
choke tube depends on the amount
of depression, and this in turn is in-
fluenced by the position of the choke
and throttle valves. The volume of
fuel can be altered to suit different
operating conditions by means of the
adjustment screws in the idle and
main jets.
43