There's a little tab alongside that holds the black cover down. Loosen the tab and you will have to rotate the cover one way or the other. One way puts tension on the bi-metallic spring inside, the other relaxes tension. Sorry I can't tell you which way to turn it, but it will be obvious when you try. This is similar to adjusting the choke, where you turn it to tension the spring so that the choke is closed when the engine is cold.
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