[Decolliber]

I had the same problem when I recently adjusted my valves, and several times had to resort to loosening the adjustor nut, opening the gap and then closing it again. Some time ago John Walker gave the following helpful tip: place the tip of the feeler guage as far down the valve stem as it will go, then slowly pull it upwards until you hear a slight click, and that may be the gap!
Checking my gaps both with the standard tool and with Island's tool, they don't seem to produce exactly the same gap. If you don't already have it, get the Island tool; it is a lot easier to use.