I have a briggs and straton engine that starts then stops . I pump gas in it and it runs but only for a few seconds, 5 -10... until the gas I pumped in it runs out. If I continue to pump gas in it it stays running. What could it be? How do I trouble shoot this problem?

Hi,
Your main jet is partially plugged. Often it is built into the bolt that holds the float bowl on and can be cleaned by removing it. Other times it is in the side of the piece that goes from the bottom to the top of the float chamber(fuel pillar.)
To explain, your pumping the primer bulb is forcing fuel into the carburetor venturi to mix with air. Not enough fuel is passing through the main jet by atmospheric pressure to sustain the engine's operation. Normally atmospheric pressure pushes on the top surface of the fuel in the bowl and is drawn upward by the lower air pressure in the venturi. This is done because the venturi is smaller than the size of the throat of the carb which speeds up the speed of the airflow which drops the pressure. The fact that you get a few seconds of run time means it is partially plugged but cannot draw fuel fast enough. On today's engines that have to meet EPA regulations, the jetting circuits (yes, there are more than one) are fixed and have to be clear since no adjustments exist to any extent. The electronics contained within the ignition module do the rest by changing the ignition timing and advance to suit the operating condition.
In your case cleaning is in order. A common recommendation here is to add a fuel additive (Seafoam is a popular one mentioned here and fairly widely available nationwide in the last few years) that will combat the buildup you have.
Peace,
Clarke

Fantastic! Thank you very much.! I will clean that as soon as I can !