[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] [New Search]

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

On 12 Feb 2005 at 22:44, Dave Hall wrote:

> 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 :-

I've got a copy of that, but I have to admit that I've not studied it.

> " 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.

I don't get what dimensions they're measuring here. I also don't understand why 
the diameter of anything would really matter, because this is situation with 
virtually no flow, so conductance is not an issue.

> 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. "

If this was written about the '72 dual advance/retard FI system, I'd be able to 
say that it was simply incorrect. Since I've never worked on one of those dual 
carb systems, all I can say is that I'm very skeptical. This explanation makes 
no sense to me, and I don't think it is consistent with what Mark Seaton has 
told us about his work on these.

Dave, don't you have one of these cars? Does it work this way?

> 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.

Yes, the retard is only functional at idle. The retard ALL goes away instantly 
as soon as the throttle is lifted at all. This happens whether the advance hose 
is connected or not. As far as I can tell, the vacuum advance portion works 
exactly the same as that on a car with just the single vac can. 

> 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.

The FI can specs are in the Bentley manual. I don't have anything here with the 
dual carb specs.

Jim Adney, jadney@vwtype3.org
Madison, Wisconsin, USA

List info at http://www.vwtype3.org/list | mailto:gregm@vwtype3.org

[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] [New Search]