Just to humor some of us, how about putting a new set of plugs in it! I've spent days trying to start a couple of engines, with plugs that looked clean, had plenty of voltage, and had been checked repeatedly for correct timing, valve adjustment, etc. A new set of plugs fixed more than one problem child! If a plug has been flooded severely, sometimes it just won't fire.
Checking the valve timing is a very good idea! Properly-adjusted valves that aren't opening at the right time don't help. The "rocking" position of the intake and exhaust should happen when the piston is at, or very near top dead center. Good luck!
Jerry
