Take the carb off and look through it...If you can see light around the edges of the disc it needs to be re set...Loosen the screw and work it around until no light is showing...
Any opening at all will let enough fuel through to keep the engine idling...
There was a spring on the shaft to close the carb when it was on the original engine..If the spring has been removed and the engine run for a while the vibration of the engine causes the disc to rattle back and forth and wear at the edges..If the disc is worn far enough the engine will never idle slowly until the disc is replaced....