FUEL
SYSTEM
The engine is now ready for starting and,
46.
CARBURATION DEFECTS
after thoroughly warming up, the speed
should be adjusted by turning each throttle
adjusting screw
an
equal amount until the
idling speed is approximately
500
R.P.M.
The synchronisation of the throttle setting
should now be checked by listening to the
hiss of each carburettor, either directly or
bv means of a piece of rubber tubing held
near the intake.
The intensity of the noise should be equal
and
if
one carburettor is louder than the
other its throttle adjusting screw should
be turned back until the intensity of hiss
is equal.
After satisfactory setting of the throttle,
the mixture should then be adjusted by
screwing the jet adjusting nuts up or down
on each carburettor
until
satisfactory run-
ning is obtained. The lever tension spring
should be connected during this operation.
This mixture adjusting may increase the
engine idling speed and each throttle
adjusting screw must be altered by the
same amount in order to reduce speed to
500
R.P.M. and the hiss of each carburettor
again compared.
The balance
of
the mixture strength should
be checked by independently lifting the
piston of each carburettor no more
than
".
The mixture is correct when this operation
causes no change in engine R.P.M. When
the engine slows down with this operation
it indicates the mixture is too weak and it
should be enriched by unscrewing the jet
adjusting nut.
An
increase of engine speed
during this operation indicates that the
mixture is too rich and, consequently, it
should be weakened off by screwmg up the
jet adjusting nut. The mixture setting
should now give a regular and even exhaust
beat, it is
irregular with a
"
splashy
"
type of misfire and a colourless exhaust,
the mixture is too weak. A regular or
rhythrmcal type of misfire
in
the exhaust
note, possibly with a blackish exhaust,
indicates the mixture is too rich.
The jets of both carburettors should be
held against the adjusting nuts before
replacing the mixture control linkage, which
should be adjusted as necessary, and
simi-
larly the throttle should be held tight
agamst their respective idling stops before
retightening the folding coupling clamp
bolt.
In the case of unsatisfactory behaviour of
the engine, before proceeding to a detailed
examination of the carburettor, it is advis-
able to carry out a general condition check
of the engine, in respects other than those
bearing upon the carburation.
Attention should, 'in particular, be directed
towards the following
:-
The ignition system.
Incorrectly adjusted contact breaker gap.
Dirty or pitted contact breaker points, or
other ignition defects.
Loss of compression of one or more
cylinders.
Incorrect plug gaps.
Oily or
duty
plugs.
Sticking valves.
Badly worn inlet valve guides.
Defective fuel pump, or chocked fuel
filter.
Leakage at joint between carburettors and
induction manifold, or between induction
manifol4 flanges and cylinder head.
If these defects are not present to a degree
which is thought accountable for unsatis-
factory engine performance, the
carburettor
should be investigated for the following
possible faults.
(a)
Pistons
Sticking.
Fig.
8.
The symptoms are stalling and a
refusal to
run
slowly, or lack of power
and heavy fuel consumption.
The piston
(3)
is designed to
lift
the
jet needle
(6)
by the depression trans-
ferred to the top side from the passage
facing the butterfly. This depression
overcomes the weight of the piston and
spring (8). The piston should move
freely over its entire range and rest on
the bridge pieces (28) when the engine
is not running.
This should be checked by gentlj
lifting the piston with a small screw-
driver and any tendency for binding
generally indicates one of the
followink
faults
:-
(i)
The damper rod may be ben
causing
binding
and this
can
bl
checked by its removal.
If th
piston is now free the dampe
rod should be smightened
ant
refitted.
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