18hp Briggs engine is burning to much fuel and smells?

We have a 18hp twin cyl briggs mower engine on a countax mower and its burning at least 1-1/2 gallons of fuel extra a day. Its not the float as this operates and the oil is clean. Do you have any suggestions. I noticed that the cyl head pressure operates the fuel pump, if that diaphram has split can oil go into the cyl thay way? I hope you have other things I could try. Thanks Andy