1968 Honda CT90 spring/valve?

I have a 1968 CT90, I noticed the other day that there is a component that is like a valve or something, it is just outside of the battery compartment, I looks like a small black cylinder with a a coil spring hooked through a rod in the middle of it. When I pull in the spring the rod in the middle moves, and then returns to where it was when I let go so it is supposed to move, however the spring isn t attached to anything, it s just dangling there. What is this piece and where does the spring attach to?

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