The adjustor screws grow loose in the rocker arms with years and miles. If you can't or won't replace the worn parts, you can restore a tighter interference thread fit by removing the adjustor screws, boogering up the threads a bit with the knurled jaws of a bench vise or some carefully placed taps of a ball peen hammer, then reinstalling. It should be
difficult to turn the adjustor screws in the rocker arms when adjusting the valves. More info
here.
