Onan Marquis 5000 quits after a while?

Remove the fuel line from the carb and put it in a gas can that is elevated above the carb. Try to run it. If it runs fine, it is the fuel pump. This can be confirmed by putting a cheap low pressure guage on it to confirm it is making 3-6psi.If it still shuts down, then check other things such as carb float and jets. Mine just gave me trouble. It was the metal rod in the mechanical speed control. The Metal had popped out of the plastic bracket. However, your problem really sounds like a fuel pump or carb float problem.