Both plugs or just one side? If it's just one side, I would suggest possibly the stuck float thing causing a degree of enrichening. Since you haven't done anything to it, that's about the only fuel related thing that happens on its own (or going lean from a clogged filter). If the ignition were somehow misfiring or retarding (insert joke here) from a failing component, I would think it would run noticeably crappy (although not necessarily so).
