Rolls-Royce
Silver
Shadow
8
Bentley
T
Series
Workshop
Manual
(b)
An
air leak at one or more of the following:
Float chamber lid joint.
Float chamber vent and drain pipes.
Weakener cut-off valve pipe unions.
Vacuum take-off plate flange on
'B'
bank
carburetter.
(c) A dirty or faulty float chamber drain valve.
(d)
Low
engine temperature, below 18°C. (64°F.)
or a faulty weakener cut-off valve.
A
high reading may be caused by:
(a) An obstruction in the weakener
air bleed
orifice or the weakener hoses.
(b) A blockage in the weakener filter.
Tuning procedure
9.
Tuning operations may now be commenced and
must be carried out in the shortest time possible. If
the time for- setting exceeds a three minutes period,
open the throttle and run the engine at 2
000
r.p.m.
for
)
minute then resume tuning. Repeat this clearing
operation
if
further periods of three minutes are
exceeded.
Immediately after a clearing operation when the
engine is at idle speed, the suction chamber should be
tapped all the way round with a light weight, non-
metallic instrument
(e.g. the wooden handle of a screw-
driver) to eliminate piston hysteresis (see
Fig.
K23).
10. Remove the pipe from the hot idle compensator
valve connection marked 'OUT'. Blank off the pipe.
11. Set the idle speed to 600
r.p.m. by adjusting the
fixed throttle stop screw.
12. Run the engine at idle speed then balance the
carburetters using the volume screws; the carburetters
are balanced when the
hiss heard at the small drilling
in
the neck of each suction chamber (see
Fig.
K23)
is
of equal intensity.
A rubber or plastic tube of approxi-
mately
4-
in. (3,17 mm.) diameter bore and
2
ft. (60,96
cm.) long should
be
used for
this
purpose.
Fit
the
dampers.
13. Turn both jet adjusting screws by equal amounts
in the same direction, approximately of a turn at a
time until the maximum
r.p.m. is recorded on the
tachometer.
Note Turning the screw clockwise
richens the
mixture conversely turning the screw
anti-clockwise weakens the mixture.
14. Set the mixture balance on each carburetter
individually using the jet adjusting screws approxi-
mately
4
of a turn at a time in either direction until
16. Check the idle speed and if necessary re-adjust to
600
r.p.m. using the fixed throttle stop screw.
17. Check that the carburetters are balanced by
raising and releasing each carburetter piston lift pin
in turn then comparing the engine response. The
car-
burettera are balanced if the response is the same for
each carburetter. If the carburetters are not balanced
repeat Operations
9
to 16 inclusive until a satisfactory
balance is obtained.
In certain countries the exhaust emission C.O. must
be checked.
If
these regulations apply Operations 18
to 21 inclusive should be carried out.
18. Purge the engine at 2 000
r.p.m. in Neutral for a
period off minute (see Operation
9).
19. Ensure that the engine has run a minimum period
of
25
minutes since the thermostat has opened (see
Operation
4)
then fit the probe of a C.O. meter into the
exhaust pipe. The C.O. meter should be set in accord-
ance with the manufacturer's instructions.
Note Suitable C.O. meters are:
1.
Horiba Mexa 200.
2.
Bosch Model Efaw 109.
20.
Idle the engine until a steady C.O. reading
is
obtained (minimum time
4
minute) then check the
exhaust emission on the C.O. meter; the correct read-
ing should be between 4
"/,
and
4$
%
(Europe only).
If the C.O. meter reading is not within
this limit, it is
permissible to unlock the jet adjusting screws and turn
them a maximum of
4
of a turn, either clockwise
or
anti-clockwise, whichever is necessary to give the
correct reading on the meter. Do not turn
then1 in
opposite directions
(i.e. richen one and wcaken thc
other). Lock the jet adjusting screws.
If the correct C.O. meter reading is unobtainable at
this setting, and settings have been carried out to the
instructions given, the carburetters should be removed
from the engine and overhauled as
dcscribed
previously.
21. Remove the C.O. meter.
22.
Remove the manometer from the float chamber
pressure tapping. Fit the cap to the pressure tapping
using a new washer.
23. Check the idle speed
atld using the fixed throttle
stop screw, reset to
600 r.p.m. if necessary.
24. Re-connect the hose to the hot idle compensator
valve.
Cold start 'fast-idle'-To check
-
-
maximum r.p.m. is obtained.
Refer to Page
K21.
15. Turn the jet adjusting screws by equal amounts
anti-clockwise (weaker
mixture) until the engine speed
just begins to fall, then turn both adjusting screws
&
of
Throttle damper plunger-To set
a turn clockwise and tighten the lock-nuts.
Refer to Page K21.
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