What is this thing?

I figured I would throw in some more explanation about how this works.

The container in the carburetor which holds gasoline is called the "bowl." Inside this bowl, there's a float - one or two sealed metal or plastic pieces that will rise and fall with the level of fuel in the bowl.

The float is attached to a little metal tang. In this case, I believe that tang would have a hole in it that the pictured item clips to. As the float moves up and down, that needle also moves up and down inside its "seat." When the float is at its highest, that rubber tip is against the top of the seat, closing off the passage for gasoline to enter the bowl.

As gasoline is burned, the level in the bowl drops, the float drops, and the pictured needle falls from the seat, allowing fuel to flow into the bowl.

This keeps the fuel level in the bowl at a specific height. If that level is too high, the engine will run too rich (too much gas in the air/fuel mixture), and if it's too low, it will run too lean. Low enough, and it'll stop running as though it's out of gas. Because the bowl is empty.

Gasoline gets pulled from the bowl by the intake air stream passing across a tube going down into the bowl. The intake air creates a low pressure, and sucks the gasoline out. As the gasoline comes out, it is atomized into the air, making it more combustible inside the engine.