I purchased a very clean (20K mi) 2007 Hexhead Dual Plug GS last fall and only got to put 1K miles on it before winter storage, so I didn't get to know the bike very well. I was prepping it for summer this week which included new spark plugs. With only 20K on the bike from new I doubt the plugs had been changed previously, and I wanted to upgrade to Iridium.
When I pulled the old plugs, the 2 uppers (accessed through the valve covers) were both a beautiful chalky light brown that indicates good carburation and burning. But when I pulled the lower 2 (accessed from under the cylinders), they were both sooty and black to the extreme. I was puzzled how 2 plugs pulled from the same cylinder and of the same age could be so different from one another. The only explanation I could come up with was that the lower plugs were not firing at all, and perhaps never had for the life of the bike.
My question is: would there have been any obvious symptoms or performance degradation if 1 of the 2 plugs in each cylinder was not firing? My experience during the mileage last fall was that the bike ran fine. It got terrible gas mileage, but otherwise seemed fine. With the new plugs in now it runs noticeably smoother especially at idle, and seems to have gained about 100 rpm at idle. But these could be results of the switch to Iridium.
My theory is supported by something I noticed when I installed the new lower plugs: the lower plug caps can give the false impression that they are secured on the plug when they are not. I had to play around with both of them to get them to click into place on the plug, and had actually already replaced the cover on one before I went back to check it only to find that I had not pressed the cap on all the way. Since I'm fairly certain the old plugs were original, is it possible that these caps might not have been pressed on correctly at the factory? Could this bike have traveled 20K miles with neither of those lower plugs firing the whole time?
If that is unlikely, what else could cause one plug to indicate excellent burning, and another from the same cylinder to be jet black and covered in soot? Love to get your thoughts.
When I pulled the old plugs, the 2 uppers (accessed through the valve covers) were both a beautiful chalky light brown that indicates good carburation and burning. But when I pulled the lower 2 (accessed from under the cylinders), they were both sooty and black to the extreme. I was puzzled how 2 plugs pulled from the same cylinder and of the same age could be so different from one another. The only explanation I could come up with was that the lower plugs were not firing at all, and perhaps never had for the life of the bike.
My question is: would there have been any obvious symptoms or performance degradation if 1 of the 2 plugs in each cylinder was not firing? My experience during the mileage last fall was that the bike ran fine. It got terrible gas mileage, but otherwise seemed fine. With the new plugs in now it runs noticeably smoother especially at idle, and seems to have gained about 100 rpm at idle. But these could be results of the switch to Iridium.
My theory is supported by something I noticed when I installed the new lower plugs: the lower plug caps can give the false impression that they are secured on the plug when they are not. I had to play around with both of them to get them to click into place on the plug, and had actually already replaced the cover on one before I went back to check it only to find that I had not pressed the cap on all the way. Since I'm fairly certain the old plugs were original, is it possible that these caps might not have been pressed on correctly at the factory? Could this bike have traveled 20K miles with neither of those lower plugs firing the whole time?
If that is unlikely, what else could cause one plug to indicate excellent burning, and another from the same cylinder to be jet black and covered in soot? Love to get your thoughts.
