Could be one or both valves aren't fully closing on #1.
Bring 31 cylinder up to TDC and the BB lined up on the timing pointer. This will ensure that both valves on #1 are fully closed.
Now check for rocker freeplay. You should easily feel the free play on both rockers. If one is tight, it could be holding a valve open. Loosen it if it is tight until you get some freeplay.
You can set the valves cold, just add approx .004 to .005 to the proper warmed up settings.
I had a similar problem on my old 216. I had 3 cylinders that wouldn't pump up [ast 40 psi. Ended up being badly burnt and worn valves and valve seats. Had the head rebuilt, it runs great now.
Good luck
Dave
