With the rocker removed and the piston down from the top of the stroke, try lightly tapping the top of the valve stem with a brass or plastic hammer while keeping it soaked with penatrating oil. A little heat on the head might help. It takes quite a bit of working it up and down even after it returns to the seat to get it completely un-gummed. I've had to do this a few times on 216 engines. I've found that if gas is much more than 6 months old, I'm ahead to drain it and rinse the tank before even trying to start it, even with Stabil in the tank. Last fall I had to unstick a valve and replace a push rod in my old army truck after I had swapped in the 216 from a '41 Chevy truck. It had not been run for a year or so before the swap. I plow my yard with it and It's ran great all winter with out having to remove the head.
