How do these valves work?

I'm trying to adjust the valves on my cast iron baseboards (Weil-Mclain I think) and either their broken or I'm not doing it right. It's a hot water monoflow tee system. The valves are stamped "Welsbach" and they don't seem to open or close, they just keep spinning in either direction. The cap isn't broken, the stem is turning, it just doesn't seem to be doing anything. They have little metal stops on the casting and a metal hook attached to the stem that catches on them. They can be turned past the stops, but you can turn them forever and nothing happens. The first one I assumed was broke, but they all are like this. Am I doing something wrong?



I've attached some pictures. I circled one of the stops on the casting in red. There's another one, but it's behind the valve so it can't be seen. The little metal hook attached to the stem that catches on the stops is circled in green. I appreciate any help anyone can give me.