Does your carb have an opening to where the adjusting tool Merc part number 91-866201 would slide in so you can turn it to the 3 1/8 turns out from lightly seated like described in Merc service manual #41 page 5B-15 that I hope you have/found on the internet?
In the photo below, you might be able to just see the center knob screw...Idle mixture screw.