Stihl 009 User Manual page 44

STIHL 009, 010, 011
Four basic operating conditions are
described below to explain the func-
tion of the carburetor:
1. The choke valve is closed and the
throttle valve partly open during the
starting process. A powerfull va-
cuum is created in the choke tube
during the induction stroke because
the entry of outside air is almost com-
pletely restricted by the closed choke
valve. This means that the engine
draws in a large amount of fuel
through all the jets and relatively little
air. A rich starting mixture is obtained
in this way. The choke valve must be
opened as soon as the engine fires -
the mixture would otherwise be too
rich and stall the engine.
2. Very little fuel is required for
idling. The choke valve is fully open
and the throttle valve almost
completely closed. The vacuum only
acts on the primary idle jet so that
fuel is only drawn off through this jet.
Owing to the pressure difference
between the choke tube (venturi) and
the intake pipe behind the throttle
valve, air flows through the
secondary idle jet and the air
correction jet into the idle chamber
and prevents the mixture becoming
too rich. However, because of this
pressure difference it is necessary to
prevent supplementary air getting
into the diaphragm chamber, making
the mixture too lean and causing the
engine to stall. This problem is
eliminated by a small plate in the
valve jet which closes against
Top:
Starting position
Bottom:
Idle position
1 - Pulse nipple
2 - Inlet valve open
3 - Fuel intake
4 - Choke valve
5 - Valve jet
6 - High-speed adjustment screw
7 - Pump diaphragm (intake position)
8 - Outlet valve closed
(both adjustment screws are drawn offset by 90°; choke valve is shown integrated in
carburetor only for this description)
the inlet when there is insufficient
depression in the choke tube.
3. During the changeover from idle
to part or full-throttle sufficient fuel
must be drawn in with the suddenly
increased flow of air when the throttle
is opened. This is effected by means
of the secondary idle jet which is
exposed to the effects of vacuum at
this point, thus producing the richer,
igniteable mixture required.
Top:
Changing from idle to part or full-throttle
position
Bottom:
Full-throttle position
9 - Throttle valve
10 - Secondary idle jet
11 - Primary idle jet
12 - Low-speed adjustment screw
13 - Metering chamber diaphragm
14 - Metering diaphragm
15 - Connection to atmosphere
16 - Inlet control lever
17 - Inlet needle
4. Opening the throttle valve further
brings the main jet (valve jet), loca-
ted at the narrowest point of the
choke tube, into operation and
provides the fuel required for
full-throttle operation.
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