The bolt is right hand thread, and loosens turning counter clockwise. After a few turns, an internal left hand thread is supposed to engage the automatic advance unit (AAU) and pop the AAU off the shaft.
It is common for the left hand thread to strip. When this happens, the bolt usually becomes captive, and no amount of turning will get it to come out.
Proceed as follows:
Loosen the bolt until there is about a 1/16" - 3/32" gap between the bolt head and the AAU.
Apply a gear puller to the sprocket, and push on the bolt head. The clearance is tight. It may be necessary to grind one leg of the puller to fit it between the timing case wall and the sprocket. Avoid pulling on the steel ring part of the AAU.
It helps, with tension on the puller, to give the sprocket a rap with a steel hammer and drift punch.
Slick