Community Forums:

My understanding is this:

At small throttle openings the intake charge is routed past
the exhaust manifold area facilitating the rapid warm-up and
easing cold start / cold running by insuring that the gasoline
is completely vaporized prior to going into the cylinder. At
larger throttle positions the secondary butterflies are held
open allowing the charge to directly enter the cylinder.

This system causes a couple of problems:
not all carbs have the arms to open the secondary butterfly

the secondary butterfly shafts wear causing an intake leak

at partial throttle the butterflies still obstruct the intake

the system is always on so the rapid warm up isn't useful if it
is really bloody hot out so heating the already hot air with the
exaust manifold only makes the car run hotter and reduces the
power of the motor because hot air is less dense than cold.

I'd take them out. They certainly didn't preserve the system
when they went to fuel injection.

chris