Red adjustment knob on 2-4 hp?

[Wee Hooker]

Ok, I'm having a complete memory lapse here. There is a red plastic, spring loaded adjustment knob on the right (tiller) side of these small outboards. Turning it at most RPM's doesn't seem to effect performance at all. I'm thinking this is a spark advance adjustment but can't recall ( and both my manuals arn't telling me!)
Can anyone shed light on it's function? Where it should be set as a "start point"?
adjustment procedure?
I'm trying to tune my 98' 4 hp Evenrude and havn't seen this adjustment since I sold my 2 hp Johnson many years back.
ANY info appreciated!