I hadn’t realized that you had the gear case apart. This makes me even more sure that the cam position is the problem. Check the manual to be sure but I believe that most Mercs, with the exception of a few portable one and two cylinder models, have the cam pointing toward the left or port side, when viewed from the rear, with the detents facing you. ( facing the trailing end of the gear case.) If you put the cam back in upside down, then you’d feel the cam detents but they would be on the WRONG side. Again, you are not the first person to have possibly screwed this up.
Long live American manufacturing!
