The large stainless screw on the bottom of the carburetor is the check valve. The check valve screw has a screen around the outside, and a tiny cylinder on the inside that prevents backflow. If this check valve is not free, there will be no idle. See this website for lots of good information. http://pochefamily.org/outboard/CarbService.html
If the check valve is stuck, soak the screw in a solvent like isopropyl alcohol or acetone. Take it out of the solvent and tap on the benchtop until it comes free, repeat as necessary. Once free, you can feel the cylinder moving back and forth when shaken.
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