I tried to attach a video but the forum wont let me add an mp4.
Post it up on youtube. Link it here for us to view.
then starts sputtering and blowing black smoke.
Means a rich condition. Carb needle sticking open or choke not locked open once warm/hot. Pressure test the carb needle/seat at 7psi for 30 minutes. Many a youtube video on how to pressure test carb needles.
Pull the oil dipstick and smell for fuel. If you smell gas, report back here. AND do NOT run the engine if you have diluted oil inside the crankcase. Fix the fuel issue and dump the oil a couple times.
I would clean/renew the entire fuel system. Clean and flush the tank like a new one. New fuel line from the tank to the carb. New filter and shut off valve are good items to install. Make sure you have a solid flow of gas AT the carb inlet. Drain into a glass jar.
Guessing water in the gas. Drain the tank into a jar and look for water in the bottom. This fits your condition to a T.