My dad has had it for 20+ years and had rebuilt the motor about 5 years back. It has new piston rings and a new piston. It hasn't ran much since then and has maybe 50 miles on the new rings. I just got it running a few days ago and it's emitting white smoke out of 1 exhaust pipe and backfires in the same pipe. I just put in fresh oil the day I got it running and it already looks black. And even after repeated cleaning the new plugs are also always blackened. Any insight on what could be going wrong with it would be really helpful. I'm pretty new to working on Harleys so I don't know a lot. Thanks for your time.