Carburetor - Husqvarna 555 Workshop Manual

7.14 Carburetor

7.14.1 Carburetor design
WARNING: The fuel used in the product has the
following hazardous properties: The fluid and its
vapour are poisonous, can cause skin irritation
and are highly inflammable.
The figures accompanying this description do not
correspond with the carburettor on the product. They
show only the principle of design and function.
The carburettor is based on three sub-systems:
• Pump unit.
• Metering unit.
• Mixing unit.
In the pump unit (A), fuel is pumped from the fuel tank to
the carburettor's metering unit. One side of the pump
diaphragm is connected to the crankcase and pulses in
time with the pressure changes in the crankcase. With
the help of inlet and outlet valve in diaphragm, the fuel is
transfered through a filter screen to inlet needle.
The fuel control unit is attached to the metering unit (B).
Here the correct quantity of fuel is adjusted for the actual
speed and power output.
The mixing unit (C) houses the choke, the throttle valve
and the diffuser jets. Here air is mixed with the fuel to
1540 - 001 -
give a fuel/air mixture that can be ignited by the ignition
spark.
7.14.2 Carburetor function
The carburetor operates differently in the following
modes:
• Cold start mode
• Idling mode
• Part throttle mode
• Full throttle mode
In cold start mode the choke valve (H) is completely
shut. This increases the vacuum in the carburettor and
fuel is easier to suck from all the diffuser jets (D), (E),
and (F). The throttle valve (I) is partly open. The air
throttle valve (J) is closed.
A
B
In idling mode, the throttle valves (I) and (J) are closed
and the choke valve (H) is open.
F
E
D
I
G
Repair instructions - 35
C
H
J