14 On 1992-on models with an open-loop
catalytic converter, release the retaining clip
and disconnect the charcoal canister hose
from the side of the carburettor.
15 On all models, disconnect the accelerator
and choke cables from the carburettor
linkages as described in Sections 3 and 5.
Free the accelerator outer cable from its
bracket, then release the retaining clip and
detach the choke cable from the carburettor.
16 Disconnect the vacuum pipe from the top
of carburettor mounting flange.
17 Release the retaining clip and disconnect
the fuel supply hose from the carburettor.
Plug the hose end to minimise fuel loss.
18 Release the retaining clip and disconnect
the vent pipe from the float chamber.
19 Undo the mounting nuts and withdraw the
carburettor from the inlet manifold, together with
the throttle cable bracket, spacer and gaskets.
Refitting
20 Refitting is the reverse sequence to
removal, noting the following points.
a) Ensure that all mating surfaces are clean
and dry, and use new gaskets.
b) Tighten the carburettor nuts evenly and
progressively, to avoid possible distortion
of the mounting flange.
c) Refit the accelerator and choke cables
with reference to Sections 3 and 5.
13 Carburettor (SU HS2 and
HS4) - fault diagnosis and
overhaul
3
Fault diagnosis
1 If a carburettor fault is suspected, always
check first that the ignition timing is correctly
set, that the spark plugs are in good condition
and correctly gapped, that the throttle and
choke cables are correctly adjusted, and that
the air cleaner filter element is clean; refer to
the relevant Sections of Chapter 1, Chapter
5B or this Chapter. If the engine is running
very roughly, first check the valve clearances
as described in Chapter 1, then check the
compression pressures as described in
Chapter 2A.
2 If careful checking of all the above
produces no improvement, the carburettor
must be removed for cleaning and overhaul.
3 Prior to overhaul, check the availability of
component parts before starting work; note
that most sealing washers, screws and
gaskets are available in kits, as are some of
the major sub-assemblies.
Overhaul
Note: Observe the precautions in Section 1
before working on any component in the fuel
system.
4 The SU carburettor is a straightforward unit to
dismantle and service, but at the same time it is
a delicate unit and clumsy handling can cause
damage. In particular, it is easy to knock the
finely tapering needle out of true, and the
greatest care should be taken to keep all the
parts associated with the dashpot in a safe
place and scrupulously clean. Prepare a clean
and uncluttered working area before starting the
dismantling, and have some small containers
handy to store the small, easily-lost parts.
5 Begin by removing the carburettor(s) from
the car as described in Section 12.
Thoroughly clean the outside of the
carburettor in paraffin or a suitable solvent
and wipe dry.
6 Unscrew the piston damper assembly and
remove it from the top of the dashpot (see
illustrations).
7 Mark the base of the dashpot and
carburettor body to ensure that on
reassembly the dashpot is refitted in the same
position. Now undo and remove the securing
screws and lift off the dashpot.
8 Next lift off the piston spring and then
carefully withdraw the piston and needle
assembly from the carburettor body. Undo
the small sunken retaining screw in the side of
the piston and lift out the needle. On later
types equipped with a spring-loaded needle,
recover the guide collar from the needle, and
the spring from the piston, after removing the
needle assembly.
9 Undo and remove the three retaining
screws and lift off the float chamber cover and
gasket. The float may be released from the
cover by gently tapping out the float hinge
pin. The fuel cut-off needle valve can now be
withdrawn from its seat in the cover and the
needle seat unscrewed if required.
10 Unscrew the union nut securing the nylon
fuel pipe to the base of the float chamber and
carefully withdraw the pipe. Note the position
of the gland, ferrule and rubber sealing
washer on the end of the pipe and make sure
that the rubber washer has not been left
behind in the float chamber as the pipe is
withdrawn. If so, hook it out carefully with a
small screwdriver.
11 If there is a tamperproof cap in position
around the jet adjusting nut at the base of the
carburettor, prise it apart with a screwdriver
and discard it.
12 Release the jet link lever return spring
from the cam lever on the linkage.
13 Undo and remove the small screw or
release the clip that secures the jet link arm to
the jet. Move the jet link arm to one side and
withdraw the jet assembly, complete with fuel
pipe from the jet housing.
14 It is not normally necessary to carry out
any further dismantling of the SU carburettor.
However, if the throttle spindle, jet housing, or
float chamber are worn, damaged, or in any
way suspect, the remainder of the carburettor
may be dismantled as described below.
15 Bend back the small tab washer and then
undo and remove the nut securing the throttle
lever to the spindle. Lift off the lever.
16 Straighten the splayed ends of the two
throttle disc retaining screws and then mark
the position of the disc in relation to the
spindle. Undo and remove the two screws,
turn the disc to the fully open position and
slide the disc out of the slot in the spindle.
Note that new throttle disc retaining screws
must be used when reassembling.
17 The spindle can now be removed from the
carburettor body.
18 To remove the choke linkage undo the
retaining pivot bolt and lift it off complete with
linkage.
19 To dismantle the jet housing, first undo
and remove the jet adjusting nut and lock
spring. Next undo and remove the jet bearing
locknut and withdraw the bearing.
20 The float chamber can be lifted off after
unscrewing the retaining through-bolt.
Recover the float chamber spacer, where
fitted.
21 The carburettor is now completely
dismantled with the exception of the piston
lifting pin (omitted on later carburettors). The
pin may be removed by prising off the small
upper retaining circlip and lifting off the pin
and spring.
22 Thoroughly clean all the carburettor
components and dry thoroughly.
23 Carefully examine the throttle spindle and
throttle disc for wear or distortion. If excessive
wear is apparent on the spindle or spindle
bushes in the carburettor body, air will enter
the carburettor, altering the mixture strength
and causing uneven running. The throttle
spindle is obtainable separately, but if the
bushes are worn, a complete carburettor
body will normally have to be obtained.
24 Closely inspect the carburettor needle. If
this has not been running centrally in the jet
orifice then the needle will have a tiny ridge
worn on it. If a ridge can be seen then the
needle must be renewed. SU carburettor
needles are made to very fine tolerances and
should a ridge be apparent no attempt should
be made to rub the ridge down with emery
paper.
25 If the needle is worn, it is likely that the jet
will also be worn. If this is the case, also
renew the jet. Also inspect the outside of the
jet head where it bears against the jet bearing.
Wear can take place here due to the action of
the choke control moving the jet up and down
in the jet bearing.
26 The most critical components of the SU
carburettor are the piston and dashpot
assembly. Free movement of the piston in the
dashpot is essential for the carburettor to
function satisfactorily. The piston is machined
to very fine tolerances so that it will not touch
the side of the dashpot or carburettor body. If
wear takes place on the centre guide tube or if
deposits build up on the internal surfaces of
the dashpot, the piston will come into contact
with the side of the dashpot and will bind. This
condition is known as piston sticking. If this
condition cannot be improved after cleaning
the inside of the dashpot and the piston with
metal polish (harsh abrasives must not be
used), then the piston and dashpot will have
to be renewed. These two components are
4A•10 Fuel system - carburettor engines
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