>
> Firstly, many thanks to Mark for reinstating the list.
>
> Can anybody out there tell me what is the purpose of the vacuum unit on the
> distributer and the pipe that goes from it to the carbs?
>
I'm assuming you've got a later spit. If so it's a Vacuum Retard unit,
which retards the timing while the car is at idle to reduce emmissions.
While at idle, with the throttle closed, there is a large vacuum created
just behind the throttle plate in the carb. The tube from the distributor
attaches at this point behind the throttle. The vacuum that exists pulls out
a diaphram at the distributor which in turns pulls on the distributor shaft
to retard the timing. As soon as you hit the gas, the throttle plate opens
and the vacuum is reduced. This allows the diaphram to fall back in, and
the distributor shaft is returned to a normal position.
> My B.L. manual suggests, as part of regular maintenance, removing the pipe at
> the distributer end, placing a finger over the end (to block it) and check
>that
> the engine speed increases by about five hundred revs... Mine doesn't, why not
> and what is happening to my spit. if the vacuum unit is faulty.
>
If you don't get an increase in engine speed when you pull off the tubing,
I would first suspect leaking or plugged vacuum tubing. If you're sure
there's no problem with the tubing then you may have a faulty retard unit.
Either way, it won't hurt your car if the system isn't working, and in fact
it'll help throttle response. (With a working vacuum retard unit, When you
hit the throttle, it takes a second for the diaphram to advance the
distributor. This translates into a slight pause when you suddenly romp on
the gas from idle.)
Hope this helps,
Jeremy DuBois
Programmer/System Administrator
Thermalogic Corporation
'74 MGB, '76 Spit
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