[5th_bike]

How long do you have it "warming up in the driveway" ?
You should always try to minimize use of the 'choke', it enriches the mixture, which could be the cause of the soot - when hydrocarbons burn only partially, it's the carbon (=soot) that doesn't get burnt. Hydrogen goes first.

Start the bike and drive off, when you are driving the motor warms up faster than when it's just moving some air and mixing some oil, on the driveway. After half a mile, turn the choke down, and after one mile you should not need the choke anymore. With warm weather, half a mile total should be enough.