ENGINE
knock (piston slap) and should there-
fore be examined.
(i) Excessive clearance between piston
and cylinder sleeve due to fair
usage or to
an
unsuitable replace-
ment part.
(3)
Pistons or rings striking ridge at
the top of the sleeve after fitting a
replacement. Such ridges should
be removed before replacement
parts are fitted.
(iii)
Collapsed piston.
(iv) Broken piston ring grooves or
excessive clearance in grooves (see
page
2).
(V)
Connecting rod misalignment.
(f)
Noisy
Valve
Rockers or Tappets
Noise due to valve rockers
can
be
identified fairly easily owing to the fact
that these are operated by the camshaft
which revolves at half engine speed,
the noise
will
seem to be slower than
other engine noises. Valve rocker noise
has
a characteristic clicking sound
which increases in volume as the
engine speed rises.
Where rocker noise is caused by ex-
cessive tappet clearance, it
can
be
eliminated by the insertion of a feeler
gauge between the stem of the valve
and the rocker toe whilst the engine is
idling.
When this complaint is experienced
and is found to be caused by
incorrect
ta
pet clearance the rockers should be
a justed
as
described on page
22.
Push rod noise may
be
caused by worn
or rough rocker
ball
ins
or push rod
B
cups
and
can
be
cure by replacing the
worn or
damaged
parts.
(g) Ignition
Knock
(Pinking)
An
ignition
knock
is recognised by its
metallic ringing note,
usually
occur-
ring
when the engine is labouring or
accelmting.
The knock
can
be
caused by either
detonation or re-ignition. Detonation
is the result
0
a rapid rise
in
pressure
of the explosive mixture,
thus
causing
the
last portion of
the charge
in
the
cylinder to be spontaneously ignited,
resulting in this striking the cylinder
wall with a ringing sound
;
this noise
being familiar to motorists as
',
p~g.''
Pre-ignition may arise as a result of
detonation owing to heat generated
thereby but may also be caused by
sharp edges or points in the corn-
bustion space, and where it arises
should be treated accordingly.
When
"
ignition knock
"
is audible,
the following possible causes should be
investigated.
(i)
Excessive carbon deposits in head
and on piston crowns.
(3)
Incorrect or faulty spark plugs
causing incandescence.
(iii)
Sharp edges or pockets in com-
bustion space.
(iv) Engine overheating.
(v)
Too
weak carburettor mixture,
causing delayed combustion.
(vi)
Unsatisfactory grade of fuel.
(vii)
Too
early ignition timing.
(viii) Faulty automatic advance and
retard mechanism due to incor-
rect
or weak centrifugal control
springs.
(ix)
Hot engine valves due to incorrect
seating width, insufficient valve
rocker clearances, valve edges
thinned
by excessive refacing.
Valve of unsuitable material.
(h)
Back
Fig
into Carburettor
It is in order that with a cold engine
back firing into the carburettors may
occur, but this should cease when the
engine attains normal
working tem-
perature.
If
back firing still persists in spite of
warming up, the following possible
causes should
be
investigated.
(i)
Incorrect ignition timing.
(ii)
Incorrect wiring of sparking plugs.
(iii)
Centrifugal
or suction advance
and retard mechanism not hc-
tioning
corractly.
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