Venturi's are the restriction, or smallest part of the carb throat that the air passes through. Jets are the metering restrictions that the fuel flows through. You need a certain amount of vacuum, or manifold depression, to draw the fuel into the airstream & too much carb will allow this "suction" at wide open throttle, & at low rpm to drop to the point where it will not draw fuel. Thats what "bog" is when you stab your foot in it off the line. All air, no fuel. Plain air is hard to light.
Chris