I've done a lot of searching and the shift drum stopper seems to be a common problem when the old ATC's get stuck in gear or won't shift out of neutral. Mine does t seem to be broken, or loose. The problem seems to be where it is sitting on the "star" shaped plate. The repair manual has the roller part of the stopper sitting in between the fingers of the star plate. My stopper actually rides along the very outer edge. Is it possible that it has an incorrect stopper? It is almost like the offset in the arm isn't enough. Or, it seems that a spacer between the arm and the mounting boss could also position it where it needs to be. Pictures show how my assembly is and also a picture from the manual.



[ATTACH][/ATTACH]

 

Both pictures look identical to me. I haven't worked on an ATC gearbox but all the later Hondas have star wheels, and the stopper arm runs on the outer surface. The bolt in the centre can come loose and the pawl and lever that moves the star wheel can crack or break to give difficult gear changing.