The cam chain adjuster on my '84 xs400 maxim just has 1 bolt in the center of the adjuster, how does this style work to adjust the chain? Is it automatic or do I need to loosen the center bolt for it to snug up?

While adjusting my valves I noticed some extra play in the chain but the manual doesn't decribe how to adjust it.

I would say that the camchain tensioner on yours is fully automatic. It works on a spring and ratchet system so as the chain wears the one way ratchet will take up any slack in the chain.The bolt in the end retains the spring inside the tensiner .Tony.