It sounds like you are getting too much fuel.
Are you sure the choke is in the correct position ??... (as in ..OFF.. after it warms up?)
You MUST install an inline fuel filter.
You MUST carefully inspect the float needle valve (in the carb) for wear on its tip and bits of crud letting in too much gas...
You MUST blow high pressure compressed air through the fuel inlet to the carb .. both ways.. after removing the float and needle..
Is the main jet still in the carb? (nobody lost it before you got it?)
Then... clean the air filter in gas (don't oil the filter yet though).
All those checks and fixes are to make sure it isn't getting too much fuel.
Then put a new spark plug in and leave the air filter off temporarily and start it and ride it and pay attention to how it is running.. run it for a half an hour and check the plug color. .
(if you live on a dusty dirt road... make sure that dust isn't going into the carb.. if it is.. you should put the air filter on for the test)
Try all that.