My 2003 GL1800 has just shy of 35000 miles. I did the second spark plug change out for new plugs at 30,000 and just changed them again (with new) at 34,800. I always use the standard NGK BKR6E-11 plug. My memory of the past times was that the plugs looked exactly as expected, first at 14000 and the next ones at 30000. The bike has gotten a little weak and seemed tired. I put new plugs in after only 4800 miles, and it is like new again. I have checked the valve clearance at 30000. I am a little overdue for my second air filter change. Question is - these plugs had a lot of black (dry) carbon deposits, unlike the last two changeouts. I recently traveled to New Brunswick and Nova Scotia and felt some gas fill-ups were not up to our quality, and I changed over to a higher octane for the remainder of that trip. Would higer octane cause carbon builup? Would poor gas cause carbon builup? I know a dirty air filter could make the mixture a bit rich and cause carbon build up. Any other thoughts on the cause?