Only thing I see that John didn't mention likely doesn't apply unless the tractor is new to you. I have seen the fuel selector valves have the lever be attached in the wrong position. That can make "on" not really be "on" & "off" not really be "off"! You MAY actually be turning the fuel off when you THINK you're turning it on and so it runs out soon as the bowl is empty. Been there!
I agree with John der fuel flow.... she's restricted some place!
Hope that helps.