Black smoke, bogs down

[forever young]

Just lately I have noticed that when I start the engine alot of black smoke comes out, I rev it about 30 seconds and it dissapears. When I start to go at slow speed there is no problem, if I try and accellerate it bogs down the more I increase the throttle, If I continue to push the throttle down it hesitates, bogs down and then after a while it takes off,planes, cruises just great and I dont have a problem all day.

I am thinking that this could be the automatic choke staying on to long. Can anyone confirm this or have any other ideas.

If it is the choke, how do I adjust it?