Black smoke = rich gas/air mixture, gray or white smoke = burning oil, so it sounds like you might have worn rings or valve guides or valve guide seals that oil is getting past. Hopefully, you don't have a cracked piston. Try running a compression check and see if all the cylinders are close to each other in compression. You might also want to check the float level in your carb and make sure it's at the correct level and while you have the carb apart, check the jets or metering rod(s)and make sure everything is ok there.
