The rear cylinder and its carburetor sound OK. The front cylinder is running rich. Check its air filter, if the two carbs have separate filters. Check that the enrichener is operating correctly. If the bike has true dual exhausts with no crossover or connection between them, the exhaust for the front cylinder will be sootier than the rear cylinder's pipe. Once the easy stuff is checked, it is time to look into the carburetor. It could be jetted incorrectly (compare jet numbers to the jets in the rear carb bearing in mind that sometimes correct jetting differs between carbs) or, if you are lucky, a jet may not have been tightened correctly and may have fallen out. Float levels are also important, and depending on the setup it might be easy to check them with no more than a couple of short pieces of clear tube.