That plug is the throttle shaft end, isn't it? It could be excess fuel running out after the engine is shut off. The throttle shaft bore or shaft end could be worn, easy enough to check, grab the throttle shaft and see how much movement you have. New bushings may help. The accelerator pump sprays fuel directly on to the shaft and blade. Modern fuels will seep out of the smallest of places and look like a giant leak, they also small like giant leaks.
Excessive idle rpm most of the time is from a vacuum leak. Check from throttle shafts down, everything from manifold bolts to wiper motors.
