Andy,
What you are looking at is yet another example of MG ingenuity....
or bodge depending on your view point ! The switch is for the transmission
controlled advance and is supposed to use 4th gear (on an '78) to gate the
vacuum line from the intake manifold to the advance unit on the distributor.
When it's working properly you can sit at idle, select 4th gear and hear
the revs build ! Since the effect is only loosely emissions related a lot
of people just connect the vacuum line directly, bypassing the switch. On my
car the wires are combined with the harness for the over-drive.
Andy McLean (also a '78 MGB)
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