On your original configuration try swaping out the Ball valve and Venturi. Also reverse the two sections of hose on the bye pass end for end. Short section+Venturi+long section. Then re-adjust the ball valve till you begin to draw gas. Again, placing the whole bye pass assb. closer to the pump may improve line pressure. If you're not seeing enough bubbles entering the tank place the assb. next to the lilly pipe. Depends on whether you want disolved Co2 or micro bubbles. HTH. Prof M

P.S. which model Mazzei Venturi are you using ?