I started my sportster last week ( in 30 degree weather) and a puff of black smoke came out of the rear exhaust pipe. When I looked further I noticed the inside of the whole pipe was black, but the front pipe was clean. I never noticed this before, should I be concerned? The bike only has 3000 miles on it. The exhaust is baffled straight pipes. I had the bike services a month ago and nothing ws found. Dyno reading was 73 in-lbs of torque. Any thoughts?