My usually reliable Simplicity Legacy won't run. This happened suddenly after a hard day of tree work. The engine turns over, but will not start. It will start briefly with starter fluid in the carb, but stops immediately. Seems to be starving from no fuel. Obviously has gas in tank. This happened very suddenly. I had just driven the tractor 1/4 mile to the job site with no apparent problems. I also maneuvered it around the job site several times just prior to having this problem. Any ideas? I'd be grateful for any help.