When an engine sits gasoline evaporates out of the carb.
As it evaporates it leaves behind varnish that will clog the tiny passages and gum up the needles in the carburetor.
Most times the only way to make it run as it should is to tear it down and clean it with a good carburetor cleaner. It's best to buy a rebuild kit for the carb to replace all the gaskets, needles and seats when you reassemble it.
Rick, March 2006