I know this is an atv site but I have a 1973 John Deere 400 and it'll fire and run for about 30 seconds with the throttle wide open and quit with pouring a little gas into the spark plug head, my guess is it's not getting any fuel to the carb but idk how to fix that problem, it has a vapor return line that's supposed to push gas into the fuel line to the carb because the fuel is vacuum fed instead of gravity fed. Plz help!!

Would it help if I took the tank off and gravity fed the carb?

 

First thing i would do is give the carb a good cleaning and replace vacuum and fuel lines.