Hi James,
I am certainly no mechanic but...I have a 1980 Merc 200. I purchased a used Seloc mechanics manual when I purchased the boat/engine last year. If I did this right, there should be two links to pics I took of of the idle speed pages. There is a idle speed screw on the port side of the motor. It is in the bottom of the first attachment. Hopefully this helps.
Duke
https://imgur.com/a/8N81dNr
https://imgur.com/a/OmTyZnA