Good work, Blaineg! Not that hard a job once you get in there, is it? Satisfying when successful, too!
Old fuel oxidizes and becomes a varnish on any fuel component. Not a nice varnish, but a weak, gummy, sticky, smelly tan coloured gunge.
On any small engine I work on that has varnish in all the lines, I'll usually just replace those fuel lines rather than try to clean'em. I've felt it's 5 bucks well spent. Definately, though, not something to overlook!