Have been looking at a parts list for a ym165 and it shows a bolt looking just like yours that screws into the side of the hydraulic control valve. Go to figure 35 if you want to look at it. It is called an adjusting bolt. Part #194200-46121. If yours screws into the side of the tranny I suppose that is not it. From your picture I can't tell for sure where it is. If it were me and having your troubles I would have no problem in slow experimental movement of it. http://downloads.pfgaustralia.com.au/intranetwebcache/Parts/Parts Manuals/Yanmar/Y00T2391.pdf