Robert Allen wrote:
> (snip)
>
> By golly, you're sure right. MGs use manifold vacuum for their
> distributor and the carbs are CD -- constant depression (or continuosly
> drips) so there will always be vacuum on each side of the throttle
> plate. Curious. That would imply that vacuum advance contributes to
> total advance even under periods of moederate acceleration. From a
> traditonal carb perspective, that is pretty strange.
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. IOW, at idle, there
is
very little if manifold vacuum available at the takeoff for the distributor
vacuum
advance. (That's also why the piston rides low at idle - there's no manifold
vacuum
available upstream from the throttle plate to lift it.) Now, when you open the
throttle, manifold vacuum is available to the distributor vacuum port on the
carb and
you get a bit of advance. Wider throttle opening, such as during acceleration,
will
increase the level of manifold vacuum to that port. This increased vacuum is
also
serving to lift the piston. Yes, the air flow has increased, but the
cross-sectional
area of the venturi has increased also, thereby preserving the "constant
depression"
at the jet bridge. The SU type carb is also known as a Constant Velocity
device -
again the reference point is the jet bridge. The air stream over that point
has a
constant velocity, but a varying size.
Bill Schooler
69 B/GT
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