Re: [T3] Too much advance

["Mark @ Mo-Sys" <markse@mo-sys.com>]

----- Original Message -----
From: "Jim Adney" <jadney@vwtype3.org>
To: <type3@vwtype3.org>
Sent: Tuesday, April 27, 2004 2:48 PM
Subject: Re: [T3] Too much advance

> > Do the vacuum cans fail by getting too soft?
>
> No, they fail by getting leaks in the diaphram, so they no longer move at
all

Thats what I thought- so unless its the wrong can there must be something
wrong with the vacuum generation.

> > Yep that was me and I'm still having troubles :-(
>
> Whoops, then it sounds like we're not to the bottom of this yet...  ;-)

Not yet- its so frustrating as I think the engine is otherwise very healthy
indeed.

> You mentioned that you have the right part #s. Are you sure? Do you
actually
> have the 311... tags on these carbs, or are you just going by the PDSIT-
> designations? Is there any chance that the tags have gotten switched, or
that
> the throttle bodies (isn't that where the vacuum ports are?) are switched?

The carbs do have the tags and I think they are original. The throttle
bodies are the originals but they have been re-built by me (but I have
always had problems with my mileage/carbs so I don't want to blame myself
for anything yet ) I have tried other bodies but the results have always
been the same.

I would like to understand more what is going on with the vacuum signal. The
port in these throttle bodies is just a small hole in the wall just below
the throttle butterfly in its idle position. As the throttle is opened
slightly the tip of the butterfly passes in close proximity over the hole
and then the hole is above the disc so I'm guessing that at that point there
should be low vacuum in the port. But at idle I would have thought there
would be quite some vacuum there? If my vacuum signal is too strong all the
way through the rpm curve it suggests to me that there must be some kind of
restriction above the carbs that is creating an unnaturaly high depression
within the carbs, or that the vac cans are too soft. I will check but I
don't see how the air cleaner could get that dirty by its design to cause a
restriction? and the air passages are so large to the air cleaner that it
would have had to suck up something huge to block them! So I'm perplexed!
Actually thinking about it I could easily check it with the aircleaner off
to see if the problem persists.

Before I re-fitted the original carbs I had some other older carbs on there
and those did an even stranger thing with the vacuum in that the advance
would rise rapidly on acceleration but then would take a long time to get
back down again like there was a vacuum lock somewhere. If I suck on the
pipe to the vac can though, I have to suck quite hard to get the advance
movement and it springs straight back when I release.

> This vacuum can just has a single vacuum connection, doesn't it?

Yes again.

The car is running extremely smoothly now I have removed the vac line-
probably the smoothest I have ever felt it so whilst I would like to really
get to the bottom of this, I'm quite happy to go without the vac advance for
now at least.

Thanks again for all your help Jim,

Mark
'73 Fasty London


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