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Re: [T3] dual vac distributor [was distributor 009]

This was something I remember reading ages ago, and I've managed to find it in
an Australian Type 3 manual by Scientific Publications, concerning the dual
vacuum distributor in the later 1600 :-

" This unit is a double chambered diaphragm assembly which is designed to
operate the distributor plate in both directions by a single arm.
There is a chamber on each side of the diaphragm.  The front chamber inlet is
0.125" diameter and is connected to the manifold below the throttle butterfly
which porovides maximum vacuum at idling speed.  The inner chamber has a 0.075"
inlet and is connected to the carburetter chamber above the throttle butterfly.

At the initial point of starting, the chamber connected to the vacuum source
below the closed throttle moves the distributor plate 7 degrees retarded.  When
the throttle is opened connecting the inner vacuum chamber to the vacuum source
above the throttle butterfly i.e. both chambers subjected to a common vacuum
source, the vacuum to the diaphragm unit is progressively equalised, returning
the distributor plate to the original static timing point.    Further
progression of the throttle opening and subsequent increase in engine rpm causes
the centrifugal advance mechanism to come into operation.
NOTE:  The vacuum unit does not advance the ignition timing.  It retards the
ignition 7 degrees at idle speed and returns the plate to the static position
after throttle opening. "

I reprint the explanation given with no guarantee this is the whole story, but
curiously in re-reading the chapter, it mentions the 2 distributor degrees of
retard on #3 cam, and warns never to use #3 for spark timing.

I've a feeling I've seen more on the dual vac unit in the official VW
publications, but I'd have to go digging as they're not to hand at the moment.
ISTR we've previously discussed special effects at small throttle openings.

I don't know for certain how the '72 FI dual vac can and the '71 twin-carb dual
vac can compare, but they have different part numbers so are not identical.

UK VW Type 3 & 4 Club
----- Original Message ----- 
From: "Jim Adney" <jadney@vwtype3.org>
To: <type3@vwtype3.org>
Sent: Saturday, February 12, 2005 4:12 PM
Subject: Re: [T3] distributor 009

> On 12 Feb 2005 at 12:42, Dave Hall wrote:
> > ... and the dual vacuum distributor uses one narrowed orifice to achieve a
> > difference between transient state and steady state vacuum response (I
> Dave, could you explain this a little more?
> On the '72 FI cars, with a dual vac advance/retard there are 2 separate ports,
> in completely different places, to feed the 2 vac chambers. The mech adv
> continues to be dependent only on rpm, of course.
> -- 
> *******************************
> Jim Adney, jadney@vwtype3.org
> Madison, Wisconsin, USA
> *******************************
> ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> List info at http://www.vwtype3.org/list | mailto:gregm@vwtype3.org
> ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

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