This is almost a groaner.
No offense here but you may be the millionth person to post here with this exact issue.
Problem is most likely the needle valve in the carb has reached the point in its life where it no longer seats and seals to the magical/mythical degree that it did when new.
The cure is to replace the needle valve and clean out the carb thoroughly.
Then you need to install a manually operated in line fuel shut off valve between the fuel filter and the fuel tank.
Train yourself to keep the inline valve shut off at all times.... EXCEPT when you are running the engine.
Your motto should be "Butt off, valve off", meaning that if you are not actually running the tractor, the valve should be turned off.
You need to correct the problem, install the inline valve, and change the engine oil and filter (if equipped) BEFORE you run the engine again, or in other words...BEFORE you RUIN the engine.
The gasoline that leaked out and into the engine has also entered the engine oil and this oil is useless now.
If you wish, you can do a topic search here on the forum for the phrase "gas in oil" or similar and read of others experiences.
Search utility is near the bottom of opening page for the forum.

This post was edited by mownie on Wed, Jun 11, 14 at 21:06

No offense taken. Should have checked before posting. Thanks for the great information.