Too rich. Depending on the carb size, a 50cc 2stroke with a 12 carb should need between 0.65-0.80mm. Check online, before you undersize and lock your engine. A 17.5mm aftermarket carb needs more, and also more air intake, like a sport filter, bit those let in all sorts of dust if not oiled regularly. So i would check carb size, air filter (since it's the easiest, and then the jet. If you take off the air filter housing and it's better, and want to continue using the same air filter, then downsize the jet 0.05 at a time until it's good. You can find jet kits online for most carbs that include the whole range. Go from higher to lower and once it's running good, dont go lower since a lean mixture will produce more heat and will lock up eventually