Have you taken the carb off the tractor & soaked it overnight in carb cleaner? (the stuff that costs $15 & comes w/ a wire basket) And blown out every passage w/ a rubber tipped air gun? If you have, you missed something as JMOR suggested. If you haven't given that carb a good soaking & blown it out, that's your problem. Replacing parts from a rebuild kit doesn't get you much.