I have a noise coming from what I think is just the left cylinder. It is a tapping sound only from 3000 to 3200 rpm. No other symptoms anywhere else on the rpm band. I checked my plug and it has black soot as opposed to the right side which is a nice grey. The top of the piston has a soft coating of black stuff which I can remove with a screw driver inserted. I have checked rocker end play and valves and they are spot on. The bike has just 15k on it and no valve recession just yet. The only thing that just might be coincidental is that 500 miles ago I added 1/2 gallon of aviation fuel for its lead content. Any ideas before I start tearing into this?