Remove and repair that petcock - they are troblesome beasts at best, so make sure it's 100% good. Pull the plugs and clean them, put the mixture screws back where they were for now. Don't pull the carbs, just drain the fuel from each seperately into clean containers and have a good look at it - is it dirty? Once the carbs are refilled, nothing leaking, you can restart it. Get it good and hot before adjusting anything. Your white smoke will dissappear, to be replaced by a wide smile If you get popping then try opening the mixture screws, then balance the carbs, and re-adjust the mixture screws for fastest idle (takes a bit of practice), you might want a fan blowing across the engine.