Message text written by "Jay Snavely"
>Could someone explain the proper source of the vacuum for the vac retard
unit. All the drawings I have seen for the 1974 TR6 configuration show the
small black vacuum line connected to the bottom of the front carb. I am
not
sure where this source is in relation to the throttle plate, but it looks
to
be upstream. It seems to me that this vacuum would be minimal at idle, and
increase with throttle opening. It seems to me that this would be opposite
of the desired vacuum for operation of the vacuum retard.
<
Jay,
In fact it is exactly the opposite. Since the throttle plate works in the
reverse direction from that in an SU carburettor the vacuum advance port
would be located on the top of the carb and the vacuum retard is located on
the bottom. The bottom of the throttle plate moves inward (downstream) as
the throttle opens and this places the port after the plate when the
throttle is closed (vacuum) and before the plate as it is opened (no
vacuum).
Note that the EGR is piped to the port on the top of the rear carb. This
applies vacuum when the throttle is open but not when it is closed (same as
for the vacuum advance, if you had one).
>There are also vacuum sources on both carbs connected to the larger, green
>vacuum line, and a port on the manifold side which connects to the EGR
>valve. What is the vacuum strength in relation to engine speed on these
>ports? What is the purpose of the green tube which runs to the TVS?
Are you talking about the lines that go to the over-run valves? I seem to
recall (I traded off my 74 some years ago) that the overrun valves are
piped through the TVS but I am not sure why. The other thing the TVS does
is remove the vacuum signal from the vacuum retard if the engine reaches a
predetermined temperature. This will increase the idle speed and hence the
cooling fan speed to improve cooling (oh, well, better than nothing).
Hope this helps
Dave
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