Link and sync

I have written before for a different problem on a 48 hp Johnson. This is another question. I adjusted the max timing using the Joe Reeves method. Deducted 4 degrees from specs. 19 to 15. After adjusting the linkage under the flywheel and returning back to normal idle position the idle adjustment screw had to be adjusted all the in to achieve idle and it is at 0 TDC at that point. Why would the idle screw have to be all the way in. That’s probably an inch and a half of threads with no more adjustments to be made in the future. Thanks for any information.