hi i have the k900 v8 version and i notice that in normal mode if i punch the gas the car would leave a cloud of black smoke or if in sport mode it leave some black smoke at high revs about 5k to 6k rpms. i wonder if anybody else notice the same or if it is normal? i use shell 93 octane
good observation. I would believe this would be "soot" as it is know around the car world that is produced by gas direct inject (GDI) engines. There is nothing wrong but it can stain the bumper of lighter color cars. Usually a little soapy water and it will come right off.
I wouldn't think a K&N air filter would make a difference in the air/fuel ratio. Everything is controlled by the engine control module. Even if you took the air filter the ECM still determines how much fuel to inject. Allowing more air into an old carburetor engine will change the A/F ratio but the K900 has a direct injected engine.