elementary manifold vacuum question

[Greg Hermann]

>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.

It would depend on the carb model, of course, but most of this sort of
Weber downdrafts are PROGRESSIVE two barrels--a small primary, and a larger
secondary, which only opens at higher loads. Sort of like the secondaries
on a Holley four barrel. The two barrel Dellorto on the Mini is like two
separate one barrel carbs which simply share the float chamber. With it,
flow to the two intake ports is completely separated, excepf for the goings
on in the balance tube, if any.

Regards, Greg

>
>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/