Date: Thu, 23 Aug 2001 03:01:47 -0700
Reply-To: Coby Smolens <cobys5@HOME.COM>
Sender: Vanagon Mailing List <vanagon@gerry.vanagon.com>
From: Coby Smolens <cobys5@HOME.COM>
Subject: Re: Digijet Distributor Questions
In-Reply-To: <85256AB0.0054DCCF.00@CDCLN08.LVS.DUPONT.COM>
Content-Type: text/plain; charset="iso-8859-1"
I think you've got it hooked up correctly, but you may have it reversed in
your head. Let's see...
The advance side of the dual advance unit is the one that pulls the plate
upon which is mounted the hall sender (would have been the points carrier in
the pre-EI days) counter-clockwise when given vacuum. In order to supply
advance vacuum the source for it has to be in the throttle body just
upstream of the throttle plate. When the plate starts to open, venturi
effect occurs at the advance port (fast moving air causes pressure to drop)
creating partial vacuum in the inboard (to the distributor) advance chamber
causing the diaghram to push the timing plate counter-clockwise, against the
direction of normal rotation of the distributor, advancing the timing. This
source of vacuum is known as "ported vacuum". There should NOT be any vacuum
at this port at idle - the timing is supposed to be retarded at idle,
remember, not advanced. If you have 15" hg at a vacuum line at idle, you can
bet it's connected to a manifold vacuum source. (Or, a slight possibility
exists that wear in the throttle body or plate is allowing for constant
venturi effect at the advance port - NOT a common problem with this motor)
Since it is at idle that the retard-side diaghram needs to pull the timing
plate clockwise, in the direction of normal rotation to retard the timing,
it gets its vacuum from the manifold ("manifold vacuum" in the lingo), which
is at maximum partial vacuum with the throttle closed. It is therefore
normal that removing the vacuum source from the retard port causes the
engine to speed up since the timing, by losing the retard effect, is
effectively advancing about ten degrees. The purpose of timing retard is an
emissions related one - at retarded idle the engine runs hotter and burns
fuel more efficiently. As soon as your foot hits the accelerator pedal,
though, that retard has to vanish, and the advance has to kick in. Using the
two opposing sources of vacuum is an effective way to accomplish this. Then,
when the revs are up to around 1200 or so the mechanical advance weights in
the bottom of the distributor have enough centrifugal force applied to begin
to swing out and pull the timing plate the rest of the way toward the end of
the advance curve, at around 40 degrees.
ALSO - The fuel pressure needs to drop at idle, leaning out the mixture
slightly. At partial acceleration, it needs to rise to its max, providing
smooth acceleration. This function is achieved by applying manifold vacuum
to the pressure regulator diaghram at idle, which compresses the internal
spring and allows fuel to return to the tank with less restriction, lowering
the effective pressure in the ring main (the high pressure side of the
system of hoses that deliver fuel to the injectors). Then at acceleration,
the vacuum pressure drops in the diaghram, allowing the spring to relax, and
restriction to return to the fuel flow, which raises the effective pressure
through the neediest phase of acceleration. This means that the correct hook
up has the advance side of the vacuum advance unit connected ONLY to the
throttle body port. It's the retard side of the unit that shares a vacuum
hose with the fuel pressure regulator - they both get vacuum from the
manifold, which, since it's downstream of the throttle, has the highest
vacuum when the throttle is closed, e.g. at idle.
I hope this is at least half as clear to you as it is to me... It's late and
I can't really tell whether what I'm writing makes any sense at all.
Coby Smolens
Valley Wagonworks
VW Bus and Vanagon Specialists
"Intimately acquainted with VW Vans since 1959"
1535 SF Drake Blvd, San Anselmo, CA 94960
415-457-5628
-----Original Message-----
From: Vanagon Mailing List [mailto:vanagon@GERRY.VANAGON.COM]On Behalf
Of Jay L Snyder
Sent: Wednesday, August 22, 2001 8:40 AM
To: vanagon@GERRY.VANAGON.COM
Subject: Digijet Distributor Questions
I have an '85 1.9l with the vacuum advance/retard distributor. I realize
the two vacuum ports are connected to different sources of vacuum--the
advance (which is teed in with the fuel pressure regulator) comes right off
of the center "manifold" box and the retard comes off of the throttle body
somewhere (venturi?). I put my vacuum guage on the line that goes to the
advance port on the distributor. For some reason, the timing wants to
advance and the engine runs faster. This I don't understand at all. How
could disconnecting the advance side cause it to advance further? I have
15" Hg at this point. Yes, I have the lines correctly installed.
I would appreciate any ideas on this and possibly a quick theory on the
purpose of the vacuum retard.
Jay
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