agree... it sounds like it's running off the accelerator pump or gas that has gotten through some clogged jets, so it only runs for a short time
i think your carb is cloggorated'ed
you can probably pick up a rebuilt carb cheap, if not you need a rebuild kit and a few hours to tear it down, soak it over night, spray out all the small passages with carb cleaner, reassemble, etc
if you do that count how many turns you are using when removing adjuster screws/jets and write that part down. that way when you put it back together you can probably just start it up and go without much need for adjustment, as long as the rebuild kit has all the same sizes
