Glenn, I think we both are right! You, if you got a slow reving engine and little overlap (=good vacum signal), and me if we're talking about racing engines (like I was). Don't forget; When you "increase the air flow" (opening the throttle) you are lifting the pistons AND the needle = More fuel. Untill the vacum piston raises there won't be more air even the throttle is full open! On race motor, piston will pop up, give a LOT of fuel BEFORE the air flow catches up = Richer mixture. you can actually see this. this condition will happen on stock motor as well if you open the bottom seat "10 - edges" - about 1.5 turn. Check it out! You hear all the time; "SUs are so simple", ain't so if you start tinkering with them. Weber DCOE is a piece of cake compared to this.