If you remove the float bowl and look on the side of the carb. where the main jet is you will see another area on the side running the full length with what looks like a small BB in the end of it. inside this area is a vacuum port area with a small BB inside. When the engine is not running this BB drops to the bottom and closes a port so you have a rich start-up mixture, Once started the vacuum pulls the BB up to open the port for a leaner fuel mixture to run.
If this gets stuck it will cause a rich run condition all the time and run rough and hunt. I've tried years ago to open one up to clean and there's no way without destroying the carb. .
Take care