Sounds like a sticky intake valve. If the intake valve sticks in the open as the piston moves up and while the exhaust valve is closed, you will have a backfire through the carburetor.
Twelve years is a long time for an engine to sit. Is the there any twelve year old gas in the tank? Varnish from old gas will gum up valve guides and glue valves to the seats.
How often does it backfire? Does it seem like on cylinder?
