Resetting The Diaphragm For Contact Breaker "Throw-Over - MG MGA Twin Cam Workshop Manual

D.6 RESETTING THE DIAPHRAGM FOR CONTACT BREAKER "THROW-OVER"
If the armature and centre rod have been unscrewed it will be necessary to reset as follows:
1. Swing to one side the spring blade which carries the contact points.
2. Fit the impact washer in the recess of the armature.
3. Screw the armature into position.
4. Place the 11 guide rollers in position around the armature. Do not use jointing compound on
the diaphragm.
5. Hold the magnet assembly in the left hand in an approximately horizontal position.
6. Screw the armature inwards until the "throw over ceases to operate, and then screw it back
gradually, a sixth of a turn (or one hole) at a time, and press the armature in after each part of
a turn until it is found that when it is pushed in slowly and firmly the 'throw-over' mechanism
operates. Unscrew the armature a further two-thirds of a turn (four of the six holes). When a
new diaphragm is fitted it is probable that considerable pressure will be required to push the
armature right home.
7. Place the cast-iron body in position on the main body, taking care to see that the drain hole in
the cast-iron body is at the bottom and all the rollers are still in their correct positions.
If a roller drops out of position it will get trapped between the two ports, and this will cut a hole
in the diaphragm.
Make sure that the cast-iron body is seating properly on the main body and insert the six
securing screws. Before tightening the screws down, it is essential that the diaphragm should
be stretched to its outermost position.
This may be done by inserting a matchstick behind one of the white fibre rollers on the outer
rocker, thus holding the points in contact (after first repositioning the spring blade into its
normal position). If a current is then passed through the pump the magnet will be energized
and will pull the armature and diaphragm forward, and while it is in this position the six screws
should be tightened. Although the diaphragm-stretching operation can be effected by the
matchstick method, a special tool for this purpose is available from the S.U. Carburetter Co.
or their Distributors. The tool is a steel wedge, to be inserted under the trunnion in the centre
of the inner rocker in order to stretch the diaphragm to its outermost position before tightening
the six flange screws.
8. Finally, check that when the spring blade is in its normal position the clearance hole in it is so
positioned around the locking screw that each contact point, according to the operation of the
outer rocker, wipes over the centre-line of the other contact point and that this action is not to
one side of the centre on either contact. The width of the gap at the points is approximately
.030 in. (.76 mm.).
9. The pump should now be placed on test, using a cut-away cover to enable the contact
breaker action to be observed and at the same time to prevent the rocker hinge pin from
falling out. A test rig of the type illustrated in Fig. D.4 is advised; either petrol (gasoline) or
paraffin (kerosene) may be used for testing purposes. Test figures are given in "GENERAL
DATA'. The use of a glass tube and rubber connections between the sump and the test tank
is advised. When the pump is switched on it should prime itself promptly, and the paraffin,
which is normally used for testing, should rise in the glass container until it flows over the top
of the pipe having the lin. (4 mm.) hole drilled in it 2 in. (5 cm.) below the top of the pipe. If the
output of the pump is not up to normal the 1/6 in. (4 mm.) diameter hole will be able to deal
with all the paraffin pumped and the liquid will not flow over the top of the pipe. If a time-test is
used, 1.5 pints (-9 litre) of fuel per minute should be pumped.
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