I ran a 460 p/u once that had been sitting for a few years and the old gas gummed up the valves in the guides. I pulled the valve springs and seals and used mineral spirits/carburetor cleaner/fingernail polish remover to get them free again. Then, new gas and it is still running fine. I'm sure you know the measures to avoid dropping the valves? But, If not: I simply ran the cylinder to the top while removing spring etc., worked the valve and disolved the varnish then reinstall. Steve