I will try to explain the link and sync adjustment. The idea is to achieve the best combination of throttle and ignition advancement for best performance. The adjustment to the cam is made by the two tiny cap screws that take a 5/16″ wrench. The two screws attach the throttle cam to the underside of the magplate and they straddle the ignition advance lever.
The port side of the throttle cam has a slotted connection that allows for the cam adjustment inward and outward as needed. By trial and error, observe the ignition advancement lever/butterfly relationship to see the amount and direction of needed adjustment. Then loosen the cap screws. Adjust the cam as needed within the slotted connection, then tighten the cap screws. Next, advance the ignition lever to observe the effect of the adjustment. Repeat as necessary.
The proper adjustment for your Johnson JW model is when the ignition advancement lever is between the two marks, the carburetor butterfly just starts to turn. Focus on the shaft for the butterfly because there is a lot of play in the rest of the linkage and the butterfly is the last component to turn.
The first of the attached pictures shows the two factory marks one is under the “L” and the other is just past the “W” on the word “SLOW”. The second picture shows the advancement lever between the marks. This is the position where the butterfly shaft should just start to turn.
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