MG MGA Twin Cam Workshop Manual page 60

B
THE IGNITION SYSTElVI
While the plug is under pressure in the tester it should
be inspected for
leakage by applying oil
round the
tcrminaJ. Leakage
is
indicated by the production
of
air
bubbles, the intensity of which will scm! to indicate the
degree
of
leakage. The leakage
g;~ses
ha\'e a 'blow-
torch' effect \\hen the engine is running which rapidly
raises the temperature of the plug. raising it above its
d esigned he<tt
range. thus producing overheating. pre-
ignition, and rapid ckctrodc destruction.
The top half of the insulator is frequ ently responsible
for poor plug performance d ue to the following faults:
splashes, accumulation of dirt and dust, cracked
insulators caused
by a
slipping spanner, ovcrtightness
of the terminal!>.
Examine for
:1
cracked insulator at the shoulder and
the
terminal post a nd ri.'move any accumulations of d irt
and dust.
Section B.13
CHECKJ;\:G T HE IGNITION TIMU\G
To prevent any excessive ignition advance, the vacuum
advance pipe must be permanently disconnected and tile
m anifold u nion blanked off with blanking plug (Pa n No.
AUC 1289).
Tne
vacuum union on the distributor must
be left open to atmosphere and the micrometer scale
permancnLiy
set to
zero.
To ensure
tJta t the
ignition timing is correct tl',roughout
the
speed range of the engine, the distributor advance
mechanism must be checked with a stroboscopic timing
ligh t and the results checked against the followi ng table.
Crankshaft r.p.m. . . 2.000 3,000 4,000 5.000
Maximum advance .
24"
26 ~
26°
26°
If the advance figures arc exceeded, the static ignition
scuing rna)' be retarded to a maximum of 3° A.T.D.C.
Check the new scnine against
1 he
table.
If the
30\31lCC
is
~till
outside the limits. renew
the
dimibutor and re-check the setting.
If the
advance is
still
not withia limits, renew the
dimibutordri\·c gears (Part Nos. AEH450 and AEHSOO).
Section B.l4
:'-'EW-TYPF: DISTRIBUTOR
A
new
distributor
(Par t No. A E141 ).
having
a
roller
weight mechanism with a positive stop (to prevent over-
advance) and no vacuum advance mechanism. has been
introduced. When this distri butor is fitted. the vacuum
advance pipe must be removed and the carburetter union
blanked off with blanking plug
(Part
No. AEH 1289).
This distributor
has been fitted
to en!rines
ha vin!!
the
following Serial Nos.:
~ ~
201 1 2040 2200
10
2206
2028 2041 2209 to 2219
2038 2188 2222 onwards.
Instructions for timing the distributor are given in
Section 8 .7.
NOTICE: --This is page 8.8 from the earlier Issue 2 of the MG Series Twin Cam
Workshop Manual. Section 8.13 here gives instructions for setting ignition timing
with the earlier distributor (vacuum advance model). For that model it was
deemed important to set the maximum spark advance at higher engine speed
using a strobe light.
8 .8
There is a Confidential Service Memorandum MG301 , 24 February 1960,
dictating dealer change to the later model distributor for all Twin Cams. However,
most cars never had the distributor changed , so the earlier release page including
the technique for timing the earlier model is included here.
MOA (Twin Cam).
lssu~
2. 27765