Occam and Bernoulli in the same thread. Fun stuff here. "Vacuum" here we go. I got up early for this? OK try this. The "vacuum" we refer to is the difference between ambient air pressure and the "lesser" pressure in plenum below the carb. At W.O.T.(wide open throttle) that difference should be ...very small. The engine should be getting all the air that will flow into it. The pressure above the 'venturi" will be almost the same as the pressure is below it. The venturi still works. It changes volume to create the difference needed to pull fuel out of the bowl through the jet into the engine. Now close the butterfly. Pressure under the carb, goes way down. Difference between the air outside(ambient) stays the same, Hoo Ya we got vacuum! We would also get a lot of fuel in the engine. That is why we have all them other jets and stuff in the Carb. Enjoy.
