gee...sounds like they gave you the wrong switchbox. the Electronic advance switch box on a mechanical advance motor.... I am surprised it will even work.
You can get one of these on ebay I think with no problem.
I think your original swithbox looks like the one below...my 1988 motor has this...
OK.....its become a matter of pride now as there is no reason why I can't just live with this minor but (for me) frustrating problem.
I have a 1988 low hour 20 hp Mercury 2 cycle that has always had a stutter between 1100 and 1600 rpm that I just cannot eliminate.
By stutter I mean it seem to...