Small Engines (Lawn Mowers, etc.): B&S engine runs out of gas, gas hose, gas tank

Question
My 18 HP B&S engine runs itself out of gas. I've cleaned the tank out and replaced all of the gas hose and the filter. The engine runs good for 15 - 20 minutes then quits. When it quits the filter has very little gas in it. could there be something wrong with the gas cap that is allowing a vacuum to be pulled on the gas tank?
If not, any suggestions?
Thank you,
Dick

Answer
Yes it is most likely the fuel cap not venting.  The other things you could do is put a new diaphragm in the fuel pump and clean and install a carburetor kit.

Be sure you put the fuel filter in the right way.  Did you check for a filter at the fuel shut off there could be one there too.

Hope this helps.