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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. Dave. UK VW Type 3 & 4 Club http://www.hallvw.clara.co.uk/ ------ ----- 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 think!). > > 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 > ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ > > > >