All
Is everyone aware that there are reprints available from original SU
publications? For example britishtool.com has a reprint of AUC9600A.
It's the Su Workshop Manual and covers H, HD, HS, HS2, and HV carbs
along with L, HP, LCS, and PD fuel pumps. A copy of this or one of the
generic carb books should be part of everyone's manuals including the
factory one specific for your vehicle.
For example the following information describes the use of the spring
inside some carbs.
Blake
TYPES OF SUCTION PISTON AND PISTON SPRING
(1) Zinc die-cast pistons were used up to 1939 in the majority of
carburettors, but this metal sometimes had the defect of slowly
distorting or growing with age, leading to loosened, piston rods
(originally a tight press fit). This can be checked by holding the
piston firmly in one hand (not a vice) whilst the second hand grasps the
rod and checks for shakeif this is detected there is nothing that can
be done as a remedy and the complete piston and suction chamber will
have to be renewed.
(2) The brass piston replaced the zinc type and ran in general
production up to 1950there was of course no trouble with instability of
this metal.
(3) The current thin-walled aluminium piston used with a weak
compression spring in turn displaced both, the previous types, and can
also be used as a service replacement for either, providing a suitable
strength of spring is selected.
To select a correct spring, weigh the old brass or zinc piston
complete with rod, and then similarly weigh the new aluminium piston and
rod, and the difference in weight, in ounces, should be taken as the
required spring rating, varying if anything on the weaker side; e.g. old
piston unit weighs 10* oz., new unit 5 oz., giving a difference of 5+
oz., so use a 4boz. spring. The springs available are:
2 1/2 oz. coloured with Light Blue paint on end coil.
4 1/2 oz. " " Red " " " "
8 oz. " " Yellow " " " "
12 oz. " " Green " " " "
(4) The old pre-war thick-walled aluminium piston used without a spring,
dropping by its own gravity weight, was used generally on twin- and
triple-carburetter layouts and is now practically out of production. It
can be replaced by the current thin-walled type plus a piston spring,
the rating of which can be found as already explained in the previous
paragraph.
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