Update on my Ford 600 series (its a 640). I rebuilt the carburetor, and it definitely was the issue. Very smooth running after the rebuild, however, now (today) it just started doing the exact same thing (running rough and gas pours out if the carb, unless I tighten the shut off valve to almost closed). On the rebuild, I did not put in a new float because the old one did show any sign of leaking, but now I am wondering if I should have gone ahead and replaced it anyway. Also, my biggest concern is gasoline in my crank case. I did an oil change about a month ago and haven't really used the tractor much since, but today when I checked the oil it smelled like gas and it's very thin. I am really hoping someone can give me good counsel on this. Thanks in advance!