Thanks all. I cleaned the carb and drained/replaced the oil and filter (which by the way, what a mess. Need to get one of those quick drain tube things). It now runs better, still doesn't like full throttle but runs plenty well enough to drive around and mow. I haven't yet noticed more gas in the oil, but will certainly keep an eye out for that. I've pulled both plugs and both are covered in soot (before cleaning the carb) so I'm guessing that meant it was firing on both sides. I did put new plugs in it when I got it, so they are brand new. I'll try pulling the wires individually as well, but it's at least working better now.