Motor dies after startup.

So I put one of the rods in, but the other won't stay in. It looks like the valve is sticky (probably caused by old oil, old gas, sitting still for 5 years, or a combination of all) and goes into the open position ok but is slow to close (come up). As a result, when the cam brings the push rod down, the rocker doesn't respond fast enough, and the push rod falls out.

I've tried to soak it in PB Blaster (both from the top and through the hole where the spark plug goes) and tapping it with a hammer or hammer/punch. Still comes back up really slow.

Any advice for loosening up a stuck valve without removing the head? Where does it commonly bind up, where the valve seats or on the stem where it passes through the block?



[All this makes me wish I just dropped the damn tank, or at least emptied it while it sat. Except I didn't know any better.]