Construction And Function; Carburettor - Husqvarna 340 Workshop Manual

Carburettor

The carburettor consists of three sub-systems:
• The metering unit (A) which contains the jets
and the fuel control mechanism. This measures
out the right amount of fuel to suit the speed of
the saw and the power demand.
• The mixing unit (B) consists of the choke,
diffuser jets and throttle valve. This is where the
air and fuel are mixed to create a flammable
mixture.
• The pump unit (C) pumps fuel from the tank to
the metering system inside the carburettor. One
side of the pump diaphragm is connected to the
crankcase and pulses as a result of pressure
changes in the crankcase. The other side of the
diaphragm sucks in the fuel.
8
– English

Construction and function

The carburettor works in different ways depending
on the setting:
• Cold start mode
• Idling mode
• Part throttle mode
• Full throttle mode
In the cold start mode the choke valve (D) is
completely closed. This increases the vacuum in
the carburettor so that fuel is sucked through the
diffuser jets faster (E).
In idling mode the throttle valve (F) is closed. Air is
sucked through an aperture in the throttle valve
and a small amount of fuel is supplied through the
diffuser jet (E).