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RE: Vacuum strangeness

To: "'Warren Pruitt'" <>,
Subject: RE: Vacuum strangeness
From: "Dodd, Kelvin" <>
Date: Thu, 17 Oct 2002 08:29:23 -0700
> I was helping a friend tune his 74 roadster last night and we 
> noticed that
> when you pull the vacuum line from the manifold to the distributor the
> engine speed increased. This is the reverse of what I have 
> typically seen.
> His car had the twin su's and the mixture and synch seemed 
> correct. Any
> ideas as to what causes this?
> Warren


One of the old standbys of SU tuning is to adjust the carbs until you reach
"lean best idle".  Then richen the mixture by 1/4 turn.  Correct running
mixture is a tad richer than the highest unloaded idle speed.  Disconnecting
the vacuum line, leans out the mixture slightly and the engine will tend to
speed up a bit.

It's a bit more complicated on the HIF equipped 1974, as at idle the vacuum
advance mechanism is operating.  Disconnecting the vacuum line at idle will
retard the timing doing who only knows what to the engine speed.


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