I have reviewed most of the threads but have not found my answer. I am new to adjusting carbs. I wanted to tune in my carb now that I have added AC and things are finally running well. I just want to get to best mileage and performance. I turned all four mixture idle screws in and back them out until I got the highest vacuum reading (I think it read 15 or 16). I then adjusted the idle to 900 RPM without AC and 600 with AC running and in gear. I thought I had things looking good but when I turn it off it wants to turn over one more time. I know that the timing and everything was working fine until I started messing with it. Can someone explain what is causing the condition and what the mixture screws do when back in or out. I hate doing adjustment and not knowing what it is doing to the system. This is a SS-680-VS on a 351W.