This gizmo is a transmission controlled switch. To minimize emissions, BL
decided to only allow the distributor vacuum advance to work at road speed.
As a result, this box is installed. The wiring is a circuit that energizes
when the gearbox is in 4th gear. That opens the vacuum circuit to the
distributor to allow the advance to function. There should be one vacuum
line coming from the manifold and the other routes to the advance diaphram
on the distributor.
From: SLUPECKI A.
Subject: Vacuum lines
Date: Tuesday, April 2, 1996 3:16
I have a 78B which is pretty much stock. On the metal pedal box located
between the brake master cylinder and clutch master cylinder, there is
what I think is a vacuum switch. There are two wires coming from the
bottom and two places for hoses to attach --- only there aren't any
hoses there. I can't find any reference to this in my Haynes manual or
the Moss/VB catalogs. What is supposed to be connected and what does
this switch do?
78 MGB (in more pieces than I would like with spring