The Engine Type designations for GV400 over the years were ADAA, ADAB, ADJB, and ADJD. For the GV400K1 the designations were ADAA, ADAB, ADJB, ADJD, BDJJ and BDJM. The Engine Type designation must be on the engine somewhere. As I said before, all of them were made only in Japan.
If you go to Item 8 in the Maintenance Instructions (right near the back of the owner manual) there are detailed instructions for adjusting the clutch plate, which they say will be necessary periodically as the clutch cone wears. I'd say you need to attend carefully to that adjustment before you look further for the source of that problem, especially in view of the possibility that the previous tenant messed up the adjustment.
