I went searching and read its been as simple as the carbs needing cleaned and a sticking choke plunger in one of the carbs. To it being a head gasket. Still pull the plugs and take a look from there I would go to the carbs first for cleaning and adjustment these are the cheap things. If that don't cure it then you may want to look into the head gaskets. Does the smoke have an oder to it. coolant , oil or fuel? If your losing coolant out of the radiator or over flow jug that would be a good sign of a head gasket.