1994 Ford F150 runs rich black smoke out the pipe

All of a sudden when I started the truck it ran rough and smoked indicating a rich fuel condition. I drove the truck about 25 miles and the disconnected the battery,checked the intake and air filter wich was all good and then reconnected the battery and the truck ran normal. Drove the truck the next day and all was fine. Drove the truck later in the afternoon and the problem was back.