First place to check would be the carb idle and main circuits, possibly way too rich. Check the spark first also, and make sure the timing is right. I was having issues starting my 250r motor, it was seemingly rich, gas pouring out the exhaust, running right some times and not at all others, turned out to be a sheared flywheel key. (you would think I'd have learned this lesson from mower and kart engines, but it took a while to figure out)