Repairs oil and gas in air filter

I have 12.5 B&S engine and when I start it, it runs ok but after it runs for 2-3 mintues it will shut off. take air cleaner off start it run ok but before it shuts off gas and oil come shooting out from where air cleaner goes on. Took carb off and in back of carb there is like a air box that has a tube going to the carb, removed that and you can see the valves and that area was full of gas and oil. what would cause this to happen?

Sounds like typical carb needle valve leak. Check the oil level in the engine, and see if it smells like gas. If it does change the oil, and remove the carb, and clean, and install carb repair kit

This is not an uncommon problem, in my personal experience. Make sure to thoroughly drain the crankcase - if too much gasoline is left in there, you will ruin the bearings in short order (my neighbor had it happen to him - his thrown-rod engine is sitting in a red wagon in my driveway waiting for me to take it apart with my kids).

The solution is to install a fuel shutoff valve in the fuel line leading to the carburetor. You can buy one online for about $6-8 and IMO every mower with the fuel tank above the carb should be equipped with one.