As I recall, there are two benefits to the balance tube.
1) you get some extra airflow from the "second" carb - that is, the
front two cylinders end up breathing through the rear carb as well
as the front. Not as much, but it is a factor.
2) because of this shared breathing, you don't end up with the
air flow in the "unused" carb coming to a dead stop, which means
that you have better breathing into the valve when it finally
opens.
Vizard makes strong claims (backed up by flowbench numbers) that a
single carb manifold is much more efficient than a dual. This is in the
context of the A series engine, with its siamesed intake ports and
closer spacing, but the basic argument makes sense: if you have the
choice between a 240 cfm carb and two 120 cfm carbs on a non-shared
manifold, the engine is only ever going to be able to breathe 120 cfm
(this is a four cylinder, so only one intake valve is open at a time).
If you add a balance tube, you increase the overall ability to breathe.
Depending on the manifold layout, a shared dual may or may not be
better than a single - all depends on how tortuous the intake path is
on the single.
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