Re: [T3] Best dizzy for AT with carbs?

["Mark Seaton" <Markse@mo-sys.com>]

> > On 21 Mar 2003 at 9:38, Jim Adney wrote:
> >
> > > What you really want is ~30 deg adv going up that hill, but it would
be
> > nice to
> > > up that to ~40 once you're at the top and you can let off the throttle
> and
> > let
> > > the engine relax again. This is what the later (vac/mech adv) dists
do,
> > and the
> > > later carbs that were made to work with them had their vacuum ports
> > drilled
> > > differently so they would provide the right vacuum under the right
> > conditions
> > > FOR THOSE DISTRIBUTORS.
> >
> > It occurs to me that the late OE dual carb cars probably have dual
advance
> > dists. For this to work, these carbs would have to have different vac
port
> > drillings to give the right signals to this later style of dist.
> >
> > Can our UK listees confirm that late dual carb engines came with
mech/vac
> > advance dists?
> >
> > Has anyone noticed a difference in the vac drilling in late vs early
> carbs?

Jim and everyone,
I'm very sorry about this but please ignore everything I said in my last
post about the vac drillings. It was based on bad memory, a too hasty look
at my carbs, and I have just realised, a big problem I have with my current
carbs which might explain a lot!
I have just had a good look at the carbs in my collection again. I have a
few odd carbs here that came with very tight venturis (21.5 instaed of 24)
which I guess are not Type3 carbs at all - though they are called 32
PDSIT-2s. One of these is what I'm using at the moment as my left carb.

One of the early (type3) sets I have had the correct venturis so I swapped
them over. This one had a drilling straight through the carb wall from the
take off tube, which mated up with a hole in the venturi- straight through
to the air flow. It ALSO has a passage way leading down through the carb
wall to small hole in the throttle plate. The throttle disk partially covers
this hole when fully closed, with part of it above the disk. As the throttle
opens the disk moves above the hole (creating a strong vacuum) I don't know
the function of the upper hole though, but I guess it will diminish the
vacuum signal.

Well when I took my NOS other carb and swapped the venturis, I saw that
there was no hole in the wall to mate with the hole in the venturi. So I
drilled it! Thinking that there was no vacuum take off!

In the later carbs, there is no hole in the venturi, just the passage down
to a similarly located hole in the throttle plate. The throttle disk has a
cut out that keeps the hole fully open above the disk at throttle closed,
and gradually transfers it to the vacuum side of the disk as it opens.
I'm a bit confused now as I have quite a few carbs in pieces and there seem
to be quite a few differences. If someone else could confirm what I'm
thinking here I would feel much better!
The rectangular hole I was talking about is I think actually for the piston
in the choke mechy.

I guess I should block the hole I drilled in the body as I'm sure I can't be
getting the right vacuum signal with both holes- or try and find a good pair
of late carbs.
Sorry if I've caused unnecessary confusion- I guess I should really check my
facts before opening my mouth.

Mark Seaton
'73 Fasty, London


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