B-tension screw

I got a cheap MTB with a Shimano Tourney rear derailleur. The guide
pulley pushes against the cogs while riding.

But the B-screw is one of those kinds that (I think) just pushes against
some kind of internal spring. Or at least it's supposed to.

Thing is, I can turn that screw by hand and it doesn't feel like it's
pushing against anything for the entire range of travel. The guide
pulley doesn't drop at all no matter the position of the screw.

Ideas? Is the thing just broke? It's a little unclear to me how to peer
inside it to see what's up, or if that's a thing you're even expected to
do with a $20 derailleur. :)

-Beej

--
Brian "Beej Jorgensen" Hall | beej@beej.us