Just for perspective...
When I had my 930 motor out I did a leakdown on it. 3-5% or so on 5 cylinders, 40-50% on another. I was about to cry...metaphorically speaking of course
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The valve looked fine, but had some carbon on it. Shot some carb cleaner on it, and it was hissing/sealing poorly.
Adjusted the valve, no difference...
Took a paint gun cleaning bristle, and ran it around the valve base for about 2-3 minutes. Nothing seemed to pop up, but there was some miniscule carbon. Poured some valve cleaner onto the closed valve, let it sit for several hours... Redid the leak down, and it was perfect. Shot some more carb cleaner on the valve while closed and let it sit - no bubbling.
Moral of the story : I was shocked how tiny the imperfection in the way the valve seats screws up the leakdown...