23hp Briggs vanguard, vertical. Has 2 fittings on top of the carb. One is for the fuel line, but the other has nothing. There doesn't seem to be any vacuum or pressure on it, and plugging it doesn't seem to make any difference in how the engine runs. The engine has been retroed to a gravely zero turn, from what I don't know. It pops and sputters, and smells rich, but the carb is clean. It also eats spark plugs like popcorn, cracking the insulators and causing them to short sideways. Any ideas, And what's that fitting for??