on the Columbia, the carb is a 12mm Bing, it doesnt have a mixture screw,
that is just your idle screw, so you didnt do anything too bad there, you probably just opened up the slide all the way,
turn it out almost all the way, it'll "choke" it and help you get it started.
Also search for Sachs Decomp. Theres a ton of info on both bikes on here.
Nice bikes BTW