I can't explain it.As the valves wear the valve seat wears.The valve is moving up to the cam.So you are moving the valve down away from cam.So you hold top nut and lossen bottom nut away from cam.Fine tuning is done with adjustment nut (top).However you need to make sure you have enough threads to adjust.It could be possible that the top nut had no room to turn down away from cam.

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1999 w140, quit voting to old, and to old to fight, a god damned veteran