Lawn Mower Carburetor Problem.?

I was taking apart my lawn mower, Briggs and Stratton 6.5 HP motor. I was checking out the Carburetor and i took apart the little cup on the bottom of the Carburetor. Inside was a plastic thing that floated there, i pulled the pin off that connected it to the lawn mower, and went to take the plastic piece off, when i think something else came flying off. When i went to put the plastic piece back on it just falls right down, instead of floating there and the lawn mower does not start. Anyone know what this piece is that is missing??