Why is my 12.5 hp Briggs and stratton spitting gas out of the carb?

Not sure where you are seeing the gas "spitting" out. I found the float was sticking on my carb (12.5hp) and allowing the cylinder and crank case to fill up with fuel. It was allowing a constant stream of fuel to flow through the carb and into the engine. When I turned it over, the fuel sloshed back up through the carb. Had to drain the oil/gas from the case and pull the head off the clean up the cylinder. Of course, I also pulled the carb off and gave it a fresh cleaning, making sure that the float was no longer sticking. The mower sat all winter and apparently the fuel varnished the carb enough to "sticky" it up.