Well first, let’s be sure we understand that this knob only controls idle speed, it is not a mixture adjustment. Then, it kind of seems like you answered your own question. Screwing the plastic knob inwards extends the stop, which limits how far back/retarded the vertical throttle rod can go.
Pretty flimsy system indeed, the snap rings always snap off the cheesy but expensive plastic knob. Part of the issue is that the threads on the “stop pin” get caught under the aluminum guide forcing the knob assembly downwards making it harder to turn. The downward pressure on the know helps break the knob at the snap ring groove.
I usually grind/flatten some of the threads off the top of the plastic stop pin to avoid this situation. Some of the older models had aluminum stop pins, you could grind a few threads off those as well.
Be sure the hole for the plastic knob is clean/free from salt/corrosion build up, and lube the outer bore of the plastic knob generously. Be sure the grease fitting is free from debris so it can easily be greased in the future.
Sorry I have no pictures, but will post a link to the parts break down so you can ask more detailed questions:
https://www.marineengine.com/parts/johnson-evinrude-parts.php?year=1976&hp=9.9&model=10624G&manufacturer=Evinrude§ion=MOTOR+COVER
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