Is it from the exhaust? If it is then here is one of the few tips in trying to remove them stains, for one i would try to soak the exhaust tip in Poorboy's Spray and Rinse Wheel Cleaner, and let them soak for about 3-5 minutes. Then use steel wool ( 0000 ) to help break apart the black buildup on it using light to medium pressure till you get them clean up and follow up with use a metal polish on it. Hope this helps in cleaning it.