You need to get a good penetrating oil and put the straw in between the valve spring coils and spray the valve stem. It take a little bit at a time. You might also want to take the rocker arm off the valve, then use a small piece of wood on top of the valve stem and give it a good whack with a heavy hammer. Do this a few times then go back and start spraying the valve stem. If it moves a little don't get carried away. Keep spraying and working it.
Good Luck Don
