What is this second hose on my lawnmower carburetor?

There is the fuel inlet and that I understand, But there is a softer hose coming out of the top of the carburetor which leads to some kind of plastic deviceup to the rear of the flywheel. Hard to explain without a photo. Too late in the day for a photo.
This is a Tecumseh engine and a Yardman mower.
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