Sunbeam ALPINE I SERIES: APLINE II SERIES, Workshop Manual, Page: 102 / 413

I
st re-issue
Page 16
to cause a fuel discharge. This fuel supplements
that already metered by the main Jet, and by-pass
Jet on WIA carburettors. It is metered by a drilling
in the nozzle and is only supplied to the air stream
under wide throttle driving conditions.
The by-pass jet (See Figs. 11 and 12)
WIA Carburettors only
In this carburettor the main Jet supplies the correct
amount of fuel necessary for normal cruising speeds
at part throttle. For maximum power at all speeds
more fuel is required than that provided by the
main metering Jet. This extra fuel Is metered
through a by-pass Jet which can only pass fuel when
the by-pass valve is open.
The opening and closing of the by-pass valve Is
controlled by the action of the by-pass valve
operating diaphragm which is situated below the
bottom of a cover held by three screws to the top
part of the carburettor. This cover forms an air-
FUEL FROM BY-PASS
JET FEEDING TO MAIN
DISCHARGE JET
HAIN
METERING JET
BY-PASS VALVE
OPEN
BY-PASS
JET
WSM.124
Section C (Fuel System)
tight compartment that Is connected by a passage­
way to the engine side of the throttle valve (see
Figs. 6 and 12).
At all light running and at cruising speed the inlet
manifold depression Is high and suction lifts the
diaphragm and Its plunger extension against Its
return spring and the by-pass valve remains closed.
At about three-quarters throttle opening the engine
suction in the Inlet manifold becomes insufficient
to hold the diaphragm and its plunger up against
its return spring pressure. The spring then pushes
the diaphragm and plunger downwards, overcoming
the pressure of the weaker by-pass valve spring and
forcing the by-pass valve off its seat. This allows
extra fuel to pass through the by-pass Jet and into
the main discharge Jet to supplement the supply
given by the metering Jet. The rating (strength) of
the diaphragm return spring is very carefully chosen
a�d the spring must not be altered in any way.
STRANGLER
(FULLY OPEN)
-....,.. ........ AIR FLOW
;- FUEL
Fis, 11. By-pass Jet In operation-WIA carburettors only