THE
S.U.
M.C.2.
CARBURETTER
*
ADJUSTMENT AND TUNING
The S.U. Carburetter is of the automatically expanding type In which the cross
sectional area of the air passage and the effective orifice of the jet are variable.
The choice of the needle which governs the effective
orifice of the jet is settled for a
particular engine after considerable testing, both on the engine test bed and after-
wards on Road Test, with Premium Grade Petrols, and
it
is
not, therefore, a-common
requirement that the needle type should be changed from the maker3< original
specification. Low grade and alcohol blended petrols may require the substitution
of a richer than standard needle.
If any doubt arises as to the correctness of the type fitted, this can be checked by
first removing the suction chamber and then slackening the side needle screw when
the needle can be pulled out and
its
markings by numbers or letters checked. These
identifying letters and numbers may be rolled round the shank, or stamped on the
flat end of
it.
If, therefore, an alteration to the mixture strength
is
required this needle
alone should be changed,
as
all
jets are of standard size and
2s
THE JET ADJUSTING
NUT IS FOR SETTING THE IDLING ONLY.
It
is most important that the needle
is
fitted with its shoulder FLUSH WITH THE
FACE OF THE PISTON,
as
shown in the diagram.
When detaching the suction chamber and piston assembly from the main carburetter
body (necessary when checking or changing the needle)
it
will be necessary to
remove the OIL CAP. After the two side screws have been removed lift the
assembly off the carburetter body. This will call for a certain amount of manual
dexterity, as the suction chamber can only be lifted a limited amount. One hand
is
required to lift the suction piston upwards inside the chamber against the piston
spring, whilst the other steadies the suction chamber. The complete unit can then
be moved sideways, clear of the main instrument, but great care must be taken to
see that the JET NEEDLE
IS
NOT BENT.
When re-fitting the suction chamber and piston the procedure
is,
of course, reversed,
and the piston should be held as high up as possible inside the suction chamber
whilst the assembly
is
guided carefully Into the piston bore and jet in the main body.
A slot in the small piston diameter registers with a riveted brass guide in the body.
Tuning the carburetter, which should only be carried out after the engine has reached
its
normal running temperature,
is
confined to correct.idling adjustment by means of
the throttle stop screw, which governs the amount of throttle.opening for IDLING
SPEED. and the jet adjusting nut
(18)
which controls the IDLING MIXTURE.
Screwing this nut up weakens the mixture and down enriches
it.
NOTE.
This nut must not be forced, as this may set the jet OH centre
*
All references to numbers in the script apply to
Fig.
64
only.
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