Husqvarna K 750 Workshop Manual page 25

10
A
24
CARBURETTOR
Functional test
Pressure tester
501 56 27-01
The pump is operated with one
hand.
The pressure tester is sup plied
with an adapter nipple for small
sizes. Make the hose connection
as short as possible.
Needle valve
Reliable inspection of the needle valve's function demands certain conditions.
The clearest result is obtained from a carburettor that is drained of fuel, yet
still has some fuel residue. A completely "dry" carburettor, or a carburettor
that has not been used for a long time give a measurement result that is
difficult to interpret.
Tip when the carburettor is full with fuel: Turn the carburettor with the pulse
line upwards, as shown in the picture. Pump up the pressure to approximately
30 psi/200 kPa and let the fuel run out through the venturi under water. Not
until air bubbles emerge from the venturi, can the function of the needle
valve be tested as set out below.
Water bath
By lowering the carburettor into the water any external leakage can be
examined at the same time as the function of the needle valve. Connect a
piece of hose to the pulse line, this should come out above the water line.
Connect a thin, short hose to the carburettor's fuel pipe. Pump a pressure
to 15–36 psi/100–250 kPa, the needle valve should then lift. Air bubbles then
come out of the jets in the venturi. The pressure should then drop to approxi-
mately 15–7 psi/100–50 kPa and then drop significantly slower or stop com-
pletely. Pressure drop after 7 psi/50 kPa indicates a leaking needle valve.
If the needle valve does not open at 36 psi/250 kPa (max. permitted pres -
sure test) the needle valve has jammed. This can usually be forced open by
blowing gently through the hole for filter compensation (A).
Leaking needle valve – indications
A leaking needle valve can be seen in numerous ways. A machine that is run
for short periods is often difficult to start when it is warm, fuel leaks into the
venturi and gives too much fuel for the next warm start. With a cold start,
especially when the machine has not been used for a long period, the fuel
chamber above the needle has been slowly emptied and fuel has evaporated.
First after several attempts to start, has new fuel been pumped forwards and
the machine starts.
Pump diaphragm
Put the hose from the pulse line
into the water and pump up the
pressure. If air comes out of the
hose, the pump diaphragm
leaks.
Measurement chamber
diaphragm
Cannot be checked with a pres-
sure measurement.
Functional test
Pressure tester 501 56 27-01
The pressure tester has a pump piston
that is operated with one hand.
The pressure tester is supplied with an
adapter nipple for small sizes. Make the
hose connection to the nipple as short as
possible, this gives a clearer test result.
Pump diaphragm
Put the hose from the pulse line into the
water and pump up the pressure. If air
comes out of the hose, the pump dia -
phragm leaks.
Measurement chamber
diaphragm
The condition of the diaphragm cannot
be checked by pressure measurement.
A punctured diaphragm stops the fuel
supply.