Husqvarna 555FX Workshop Manual page 21

Fuel system
The carburettor's design
The carburettor can be divided into
three different functional units: the
metering unit, the blending unit, and the
pump unit.
The metering unit
The needles and the fuel control
functions are located here.
The blending unit
Fuel and air are mixed here.
The pump unit
Pumps fuel from the tank to the
carburettor.
The carburettor's design
The carburettor can be divided into
three different functional units: the
metering unit, the blending unit, and the
pump unit.
The metering unit
The needles (A) and the fuel control
functions are located here.
The needle valve (B) and control
diaphragm (C) are vital to the
carburettor's function.
The blending unit
In this section of the carburettor fuel and
air are mixed in the proper proportions.
The choke and throttle valves are
placed here.
In the middle of the venturi (narrowest
part of the throughput) the main jet (D)
is found.
The pump unit
This is where the pump diaphragm (E)
that pumps fuel from the tank to the
carburettor's measuring unit is found.
The diaphragm is affected by pressure
variations in the engine's crankcase via
an impulse channel (F).
If the channel is blocked, by grease or
a wrongly facing gasket, for example,
the pump unit does not function and the
engine cannot be started.
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