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THE
IGNITION
S
YSTEM
B
blttery,
faulty distributor, coil or condenser
defects,
or a
bro.
k en
or
worn-out cable
may
be additional causes.
If the plugs
appear
to
be
suiwble
for further
use
proceed
to clean and
test
them.
First
remove
tbc
plug gaskets and examine them for
condition.
A large proportion
of
the heat of the plug is
nonnally dissipated
to
the
cylinder
head through the
psi:et
berv.wn the plug and the head.
Plugs
not screwed
dowo tightly can
easily
become overheated
so
that they
oper:~te
out of their proper heat range.
producing
prt-ignition, short
plug
life,
and
'pinking'.
On the
Olher band,
it
is
unnecessary
and unwise
to
tighten
up
the
plugs
too much. What
is required is
a reasonably
good
seal
between
the
plug
and
the
cylinder head and
the
use of a
torque
wrench is recommended
to
tig
hten
the
plugs to a figure of 30
lb.
ft. (
4·1
5
kg.
m.).
II the plugs require cleaning it is preferable
to
make
liSt
o(
a proper plug cleaner of the type recommended
by
the
plug manufacturers, and the makers'
instructions
for
using
the cleaner should be follo\\ed carefully.
Occasionally a blistered
insulator
or a
badly
burnt
electrode may be noticed when
examining
the
plugs.
If the plug
is of
tbe
type normally
recommended for
the
engine
and
it
was correctly
installed
(down
tightly
on
the
gasket),
this
condition
may
have
been
brought
about
by a very
lean
mi~tturc
or
an O\'Crheated engine.
There is,
howe•·er, a possibility that a
plug
of another
type
is
required, but as
a
rule
the
recommended plug
should be adhered
to.
Aner cleaning carefully, e>:amine
tbe
plugs for cracked
wulators
and \\ear of
the
insulator nose
due
to excessive
previous
cleaning.
T
n
such cases
the
plugs
have passed
their useful life,
and
new
plugs
should
be installed.
Examine the
insulator
for deposits
underneath
the
side
electrode
which have possibly accumulated and
which
act
as a 'hot-spot'
in
service.
Aner cleaning
the
plugs in a special cleaner blow
all surplus
abrasive
out of the body recesses, and off
tbe
plu3
thn:ads, by means of an
air
blast. Next
examine
the threads
for carbon.
Any
deposits
can
be
removed
and the threads
cleaned
with a wire brush.
A
wire
buffing
wheel
may
also be
u
tilized,
but reasonable care
must
be
used
in
both
methods in order
not
to injure
the
electrodes
o
r
the
tip of
the
insulator.
The thread section
of
the plug body
is
ofien
neglected
when
cleaning the
plugs,
owing
to
the fact that
it is not generally realized
that,
like
the
gaskets,
the threads
arc a
means
of heat
dissipation and that when they are coated with carbon
the flow of the heat from the plug rs retarded, producing
0\~rheating.
This
simple
procedure
will
also ensure
absence of binding on the
threads
on
replacemerll and
also obviate unnecessary use
of
the plug
spanner.
When
replacing
a plug always
screw
it down by hand
as far as
possible and use the torque wrench for
final
MGA
('l'vUI Cam).
Issue
3.
32403
tightening only.
Whenever possible.
use a socket to
avoid
possible fracture of
the
insulator.
Examine the
electrodes for the
correct gnp
(sec
'GENERAL
DATA
'). Avoid
an
incorrect reading
in
the
case of badly pitted electrodes.
Remember that electrode
corrosion
and
the
dcveiO(>-
ment of oxides at
the gap
area
vitally affects the
SJ'I"rking
efficiency.
The
special cleaner
can
remove
the
oxides
and deposits from the insulator. but the cleaner
stream
does not
always
reach this area with fuU effect
owing
to
its
location, and cannot
nc~ssarily
deal
with
e<>rro~ion
effectively as
this
sometimes requires
too
strong
a
blast
for
proper
removal.
When plugs
appear worthy of further usc
it is
[!,OOd
practice to
dress
the gap area on
both
centre and
side
electrodes with a
small
file before resetting them to
the
COn'CC:t gap. The intense beat, pressure,
explo~ion
shock, and electrical
and
chemical action to which
the
plu~
are submitted during miles of
service
are
so
intense
that the molecular
structure
of the metal
points
is
eventually affected. Plug• then reach a worn-out
con-
dition
and
resetting the points
can
no
longer
serve
a
good purpose.
When
points
are
badly
burnt it is
indicative
that
the
plug
has worn
to such an extent that
its further
use
is undesirable and wasteful.
Before replacing the plug in the engine
tes
t
it
for
correct
functioning under
air pressure
in
a
plug
tester,
following out the instructions
issued
by
the
makers
of
the plug tester. Generally
speaking.
a plug may be
considered
satisfactory for further service
if it sparks
continuously under a pre$SUre
of
I 00 lb./sq.
in.
(7
kg./em.l)
with the gap between
the
points
set
at
·022 in.
(·56 mm.).
It
is essential that
the
plug
point should be reset
to
the
recommended
gap
before the
plug is refitted to
the
engine (see
'GENERAL
DATA')
While the
plug is under pressure
in
the
tester it
should
be
inspected
for
leakage by applying oil round the
terminal.
Leakage is indicated by
the
p.-oduction
of air
'o•••J
•
Fig. B.S
Adjusting
the
plug
gap
with the Champion
setting tool
8.7
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