Someone asked the other day about re-attaching the anti-run-on valve in a
GT6. I posted a reply and promised to look at the vacuum lines on my car.
My GT6 no longer has any of that stuff other than a gas tank vapor recovery
line, but my 1980 Spitfire has all that stuff still connected (I think). So
here is what I found. (I confess I've never spent much time worrying about
it before.)
The entire "vacuum" system seems to be connected via one of the carbon
cannisters. (The other cannister is disconnected, and I always figured it
was just a spare that the dealer was supposed to swap over after so many
miles.) There is one main vacuum line that handles PCV, connecting the
valve cover with an inlet on the carb and continuing over to the cannister.
Another line from the gas tank (presumably) also goes into the cannister.
There is only one way this could work under normal operation - the inlet on
the carb provides some small vacuum which extracts blowby from the valve
cover and also pulls the pressure inside the cannister case below air
pressure, which would extract vapor from the tank.
The a-r-o valve sits between a line going directly to the intake manifold
and yet another line to the cannister. When the a-r-o valve opens it would
expose full manifold vacuum to the interior of the cannister casing. (This
is why I conclude that the "normal" line provides only a small vacuum.)
This sudden increase in vacuum would have minimal effect on the blowby but
could have two other effects. First, the tank would suddenly experience
more vacuum, making it harder for the fuel pump to draw, not likely by
itself to have much a-r-o effect. However it would also introduce more
vacuum into the carb somewhere upstream of the throttle plate. I don't know
where that line goes in the carb, so I can only guess what it is supposed to
do. Best guess is that it reduces the airflow across the jets.
Now for the question. My GT6 has a pari of SU's. One of them has a small
leak. On one, the small wire-spring covered line running from a floatbowl
to the jet is wet, apparently split. Are these easily replaced? Are parts
avaialble? Any suggestions?
Jim Muller
jimmuller@pop.rcn.com
'80 Spitfire (Percy)
'70 GT6+ (Nigel)
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