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Re: Weber acceleration problem....

To: mgs@Autox.Team.Net
Subject: Re: Weber acceleration problem....
From: tip@ai.chem.ohiou.edu (Thomas I Perigrin)
Date: Tue, 10 Nov 1998 16:25:10 -0500
>     This carb is the same one that was on the engine when it was laid up a
>     couple of years ago.  It ran fine prior to that.  I have not changed
>     anything on it.  The only thing I have done is to rebuild and clean
>     it.  So it should not be a jet sizing problem.

>     I don't actually have it on the road yet, so all of this is coming
>     from playing with the engine sitting in the garage.  I also let the
>     engine get warm to test all of this out.  The problem is that the
>     engine spits and sputters when the throttle is opened suddenly (sudden
>     acceleration).  If I mostly close the choke plate and  hit the
>     throttle, it seems to work just fine.  So I think it is a fuel
>     starvation problem.
>
>     What I do know:
>     1. The car idles fine.
>     2. The car revs smoothly to high RPMs if taken slowly.
>     3. Acceleration seems to work fine if the choke plates are mostly
>     closed.
>     4. The accelerator pump appears to spraying fuel into the carb (I can
>     hear and see it).  Whether it is the proper amount, I don't know.
>     5.  I plugged a vacuum gauge into the manifold, and got a pretty
>     steady number at around 20, so I figure that there is not a vacuum
>     leak in the intake system.
>     6. The timing appears to be properly set (per manual instructions).
>
>     What am I missing?

Many years ago I had a perplexing problem that may be related.   I rebuilt
my twin SU's, and -

1) The car idled fine
2) The car revved to high RPM's if taken slowly
3) I had the right fuel mixture according to simple tests
4) I got good vaccuum gauge readings
5) The timing seemed to be set correctly

BUT - when I took the car out onto the road it would drive well for a
while, and then suddenly the thing would lose power and overheat
DRAMATICALLY.

I would take it back home, and #1 through #5 would still be true!

It took a LONG time and the advice of an oooooold mechanic to find the problem.

The inlet manifold has two plates fore and aft, that seal off the
horizontally bored gallery.   These plates are pressed into place, and then
the manifold is indented in a few places to cause small bits of metal to
extend over the plate, holding it firmly in place.  They are at 12, 4 and 8
oclock on my manifold.

And this was the source of the problem. When I took the car out on the
road, a road bump would cause the rear plate (the one you can't see easily)
to fall partially open.  The dimple at 12 oclock had somehow become
ineffective, and so it would fall open until the dimples at 4 and 8 oclock
caught it.   So it would be leaning open with as much as a 1/4" gap at the
top.   This greatly diluted the fuel mixture, making it far too lean, AND
it destroyed the vaccuum for the vacuum advance, changing the timing all to
heck.  So, the car was running at high RPMS both lean and retarded.  ICK.

However, when I would take my foot off the accelerator, the vaccuum would
temporarily increase, and it would suck the plate back closed.   When those
plates were in place, the vaccuum was good, and it would be properly timed
and carbuerated.   And I could rev it up, and the plates would stay in
place until there was low manifold vaccuum AND a sudden jolt.  So, as far
as I could ever tell in the garage, the thing seemed to be correctly timed
and carueratted.

Anyway, that may be related to your problem...

---

A .sig is static        My life is dynamic.     I can't think of a single
quote or witty saying that expresses the mercurial nature of the changes in
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