The float valve is probably leaking and letting fuel run down into that cylinder. Or the floats are adjusted wrong.
Pull the carbs and set them upside down on the bench. Pull the bowls off the bottom and measure the distance from the carb body to the bottom of the floats. Should be in the 24mm neighborhood(someone doublecheck me on that number). Also look for dirt in that little needle valve that the floats control.
Wait, you have fuel in the exhaust? That just sound like that plug isn't firing. You can pull the plug, plug it in, ground it against something metal to see if it's sparking. If you just let the plug dangle you could burn up your coil.