I have owned two Polaris quads which my wife has thoroughly enjoyed. First, she rode a Trailblazer, and now she rides a 2000 Scrambler 400 4x2. Both of these quads get covered in this black dust under the seat. The inside of the plastics, the gas tank, the air filter are all stained with this stuff. I am assuming that this "dust" is created by the drive belt. Is this correct? Is their anything that can be done to better contain this stuff inside the CVT cover?

 

Ya, it could be from the belt, check wear the vent tube is going. I have a similar problem, my tank has black dust on it and so does it under the front fender. I cleaned it and rode the machine again after about a week the dust was back it was from the vent tube. I'm in the process of figuring out a way to reroute the belt housing vent tube.
I'll E-mail you when I'm done for know mach up a filter at the end of the vent tube with some air filter foam that'll keep the dust from getting on your machine.
Good Luck!

2000 SCRAMBLER 400 2X4

SCRAMDADDY400