The very first thing you need to do is make sure the choke is opening up all the way. Pull the air cleaner, get it hot and look at the choke. Is it open all the way.

What type of carb does it have on it. Stock, aftermarket, Webber? Also get us a picture of whats under the air cleaner. Black smoke is over fueling. One trick is to snap rev it, then take a rag in your hand and cover the intake. This will cause fuel to suck from all the holes in the carb. Don't let go of the rag. When it slows down remove the rag and get it to idle and clear out the excessive fuel. Do that a couple of times and test drive it. You might have a chunk of rust in the needle and seat. This can sometimes blow it out of there.