Need a Expert Mechanic- 50 Mercury 4 stroke

[whopper]

I am from westcentral minnesota and have a 1998 Merc 50 4 stroke that trolls too fast.I have moved only the idle screw, but it still goes a little too fast before konking out- 625rpm is the very best when it warms up.I want to take the motor someplace that has a real good mechanic who has had experience with this problem.I will take it anywhere-up to 100 miles to have this fixed.My old 50 Mercury Classic trolled like a champ but not this one. I also bought another prop. This helped but did not fix the problem.