elementary manifold vacuum question

[Tom Parker]

Stowe, Ted-SEA <StowT at PerkinsCoie.com> wrote:

>Tom thanks for the info.
>yes this is a 5 port head, I have a pierce manifold and a downdraft Weber on
>it, the car came that way. I don't know if I have the original head or cam.
>due to the fairly awkward manual choke , when it starts up cold it runs at
>about 600+ rpm, a few minutes later it's doing it's normally 1000.

I'm running a Mini with sidedraft Dellorto. I've seen downdraft Weber's on
Mini's, I imagine the manifold is similar to yours, but perhaps smaller port
spacing.

>I did a compression test last night and it came to be
>130 130 125 135
>1   2   3   4

Good that they are closish together.

>this manifold just has 2 large 'pipes' which go to each intake port, these
>come from a pretty large base which meets up with the downdraft carb. so
>there is no balance tube as such. I am measuring this from a port on the
>manifold, but come to think of it, this is about 1 inch out of the front
>head intake port, so the other port isn't really a major part of this
>reading.

I've not looked closely at such a downdraft Weber, but I believe it has one
big venturi and a smaller one inside that? What I'm trying to say, is that it
is basically a single barral feeding both ports via a phlenum (ok, its a
clever word for an air box, and I can't spell it) in the manifold.

If this is so, then you should probably mount your gauge midway between the
ports on the phlenum.

When the gauge only sees one port, it is even worse than you might think, if
the firing order is the same as the A series. My A-series fires 1-3-4-2, or
2-1-3-4. This means that the 1-2 port gets sucked twice in succession, and
then left for a full turn of the engine. You get 1-2-phut-phut-1-2-phut-phut
on the gauge so it isn't surprising that you can see the pulses, they will be
comming in at half the engine speed.

--
Tom Parker - tparker at nznet.gen.nz
           - http://www.geocities.com/MotorCity/Track/8381/