Do all the coils on the plug glow or is the first one or two not glowing when you run a current through it? Sometimes, even if the filament is damaged, you can still run a current through and get it to glow at the bottom of the plug but it is still bad for performance/starting. Also, have you varified that fuel is flowing through the carb?
If your fuel is old I would try to get some new. You can store fuel for a year in a good sealed container in a dry, cool pace out of the sun, but fuel does go bad over time.