Someone aske if modifying a Weber to have both chokes open at the same time
would help midrange response. I don't think this will help for the
following reasons.
Carburators work on the venturi principle. An area in the carb is of
smaller cross section, the air speeds up, and the pressure goes down. A
small hole in this area lets fuel into the air stream The fuel moves
because of the difference in pressure between the faster air and the still
air around the float bowl. This difference in pressure is often called
"signal".
A venturi of any given size works best in a fairly narrow band of airflows.
If the flow is too slow, there isn't enough signal to pull fuel through
the jet consistently. If the flow is too fast, then the venturi becomes
too restrictive and limits the amount of air that can get into the engine.
This is why sizing a carb to an engine is so important.
Webers are constant size venturi carbs. The venturis are sized such that
one venturi works well for low air flows and, when the airflow reaches the
speed where restriction becomes a problem, the second one kicks in. If
both venturis opened immediately then neither would generate sufficient
signal to cleanly pull fuel.
On our beloved SU's the dashpot varies the venturi size so the carb can
generate sufficient signal at low speed while still providing enough air
flow at high power settings. Really a neat idea when you think about it.
If you tried to run a constant venturi carb the size of two SU's without
having a progressive linkage and you would loose a lot of smoothness at low
speeds. Look at a side draft Weber for proof of this.
Regards,
Bill Eastman
61 MGA with 2 SU's just like uncle Syd wanted it.
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