Those have an electric fuel pump. Maybe the float or needle valve is stuck open letting the cylinder fill up with gas. The carb has an overflow, but maybe it's feeling up quicker than it can run out and going into the cylinder. Then going by the rings into the crankcase. Just a thought. Better change the oil before you run it any more. Gas in the oil is hard on an engine.
I saw a car do this one time. It was flooding bad and the guy kept driving it. It had a lot of gas in the crankcase and one day he started it up. When he started it, it sounded like a 12 gauge shot gun went off. It blew the valve cover gaskets out from under the valve covers. It also blew the muffler and tail pipe all the way off on the ground. I would have never believed it if I hadn't seen it, myself.