Bill Schooler wrote:
>
> Bob, I've just gotta chime in here because your description of the SU carb as
>a
> constant depression device is incorrect. The ONLY place in the SU where
>there is a
> constant depression is right on top of the jet, where the venturi is formed
>by the jet
> bridge and the top of the piston. On the upstream side of that point is
>atmospheric
> pressure and on the downstream (engine) side is a varying degree of vacuum,
>depending
> upon the position of the throttle plate. If you take a real close look at an
>SU HS4
> carb, with the vacuum port on the rear carb, you will note that this port is
>just
> barely on the upstream side of the throttle disk, at idle.
Okay, okay, I'm conceding defeat!
Point 1:
On my old 1974 Midget (now gone), the vacuum advance came right of
the log manifold. Ported versus manifold vacuum was irrelevant. The
carbs had two fairly big holes above and upstream of the throttle
plates but this was used for crankcase scavenging and I never did
figure out why that was a good idea.
Point 2:
Out in the parts pile I have two grubby SUs that supposedly came from
a 1967 'B'. As Messrs. Darby and Schooler pointed out, the vacuum
advance is taken off the rear carb. The inlet is on the top of the
carb and it is directly over the throttle plates such that, when
closed, the butterfly will most surely cover the port. Also, as the
throttle is opened, the butterfly moves upstream from the port. The
port is so small only a small paper clip wire would fit through it.
This seems odd to me and, in operation, probably performs as
Mr. Gaylord described.
Point 3:
I pulled out the books on Weber carbs. The DGV downdraft appears to have
a
vacuum port just below the choke housing that would indicate it is
downstream
from the throttle plate. However the throttle plate would move away from
the
port thus shrouding the vacuum port during on-throttle transitions as in
point 1.
Point 4:
I pulled the cover off the '75 TR6 and raised the bonnet to find the
vacuum
advance is taken off the bottom of the front Stromberg CD. The port is
just
slightly downstream of the throttle plate and would operate as I have
been
preaching 'ported vacuum' all day. I was doing this inspection with the
aid
of a flashlight, wearing an MG T-shirt, with the temperature in the low
30's
and in gusting snow (I am an enthusiast!).
Point 5:
After I covered the TR6 back up and returned to the garage, while
brushing
snow off my shoulders, I gazed longingly at the C/GT with 3 sidedraft
Webers,
the Mallory dual point, and no vacuum advance provisions at all and
thought
I should quietly return to that world.
Point 6:
It's late, I'm tired, and at this point I'll call the SUs and their
provisions
for vacuum advance pretty much anything you want. I thought the 'CD'
moniker
covered the family of carbs where the throttle plate only suggested how
much
air should be allowed to the motor. That big piston thingy that stuck
the
sharp stick into the little hole where the gas dribbles out is what
actually has the final say.
After some rest, I can probably come up with the layman terms.
Bob Allen, Kansas City, "The Show Me State"
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