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Re: [T3] Some carb or ignition problems...


<x-charset iso-8859-1>I read this as meaning the manifold vacuum advances the timing 7deg at idle
but the carb vac retards it that much, leaving timing at TDC while idling
(that's why it's set at 0deg hoses on for the twin carb twin vac!).
What worries me here is that Ben is timing at TDC as is normal for that
model, but with the wrong distributor.

Can someone explain how the idle timing can be so different engine to
engine - I assumed it is to do with the advance characteristics of the
distributor, and am having difficulty understanding how an early FI
distributor will behave the same way as the dual would as both are listed as
being timed at TDC.

I've just checked the '69 AT injection Fastback vehicle analysis (by the UK
Motor Industry Research Association) and their figures show the vac advance
for 311 905 205L as 0 at 2000 crankshaft rpm (4" Hg manifold vac),
increasing linearly to about 10 deg at 3,800 (7.8" Hg manifold vac).
Centrifugal advance goes from 0deg at 1000 rpm to 20 at 2000, then maxing at
30 deg at 2800 rpm.   Fascinating figures, but does it help here?  ;-)
Clearly it's not with WOT, but partial throttle and above idle engine speed,
as you said, Jim.

Incidentally, the oil temps measured after 45 minutes at top speed (81mph)
on the high speed circuit were 116C (241F) at the sump plug, 98C (208F) at
the diff drain, and the cylinder fin (tip I presume) was at 109C (228F).
Ambient was 10C (50F).

Another thing I noticed was that oil consumption was about 4 times as much
at 80 as at 60mph, but was actually a bit higher at 40 than at 60mph.

Emissions test was 1.13% CO (2.3% permitted by California then), but at 440
PPM for HC was marginally over the Calif. limit of 410 PPm, after only 2600
miles.

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: Monday, September 16, 2002 5:43 PM
Subject: Re: [T3] Some carb or ignition problems...


On 15 Sep 2002, at 23:22, Dave Hall wrote:

> The only stock Type 3 distributors where timing is done with hose(s) on is
> the twin vac on the dual carb (ie the one yours should have).  Whether it
> makes a difference in every case I don't know, but I can't see why they
> would bother to get you to remove the hose if it wasn't desirable.

I think the admonition to remove the hoses is just left over from some
earlier
era, since the vacuum advance does nothing at idle on any system that I am
aware of that uses a dist with mech & vac advance, but not vac retard.

> NOTE: The vacuum unit does not advance the ignition timing.  It retards
the
> ignition 7° at idle speed and returns the plate to the static position
after
> throttle opening."

I assume this just refers to the retard portion of the vac adv/retard
mechanism.

> There's a bit more to the design, as the two ports have different bores,
so
> there is a gradual change to the plate position.

This is probably the key here. The interplay of the vac adv and retard is
probably pretty subtle, but I believe the vac retard goes away instantly
once
the throttle is opened, leaving the vac advance to come on slowly at higher
rpm and lower throttle. I don't expect either of them to be doing anything
at
WOT.

-
Jim Adney
jadney@vwtype3.org
Madison, WI 53711-3054
USA

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