I just adjusted my valves and had this exact problem. Took me a bit to figure out. You have to clamp the large nut that is holding the valve spring down, and hold it so it doesnt rotate (with the valve) when you turn that bottom nut. Otherwise, you don't do anything to the valve height. After figuring this out it was straightforward (other than figuring out what to use to hold this thing.) I used a wide-grip pliers, needle-nose vicegrips might work well. Use your imagination.

you shouldn't have to turn the adjustment screw much to get an adjustment. Maybe a turn or two.

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'79 240D 4 speed manual 105k miles