The venturi stack is an intake extension, like a nozzle. MOSTLY, they are to prevent back flow of fuel. They can also put the intake in a better flow of air. Depends on your arrangement. If the air intake (at to opening of the venturi stack or the carb) gets very different air to the diaphram on the side of the carb, you will get some tuning issues. There are experts far smarter than me on the subject that can be real help to you from here. FOR ME, intake extensions have not had an impact on tuning.

Good luck.

Bedford