I installed a fresh spark plug not too long ago and rode for an hour or two. I decided to clean my carb, and after reinstalling it, I had difficulty getting my bike to start; I pulled the plug and sure enough it was wet. I wiped it off and let it dry, then reinstalled it but prior to torquing it down, I kicked the bike over a few times and then went to take the plug out to check again and felt a snap. The threads of the plug were stuck in the head, and the rest of the plug pulled out. I thought this was odd because I never torqued the plug down. See the middle spark plug in this picture: I took another spark plug and cut the threads off to find out what size extractor I should use. It ended up being a #4 extractor. Since the hole is so deep in the head, you can't get a tap wrench or adjustable wrench on the extractor to turn it. I ended up using a 1/4" drive 5/16" 12-point socket with a 3" extension to turn the screw/bolt extractor. I was able to remove the stuck threads by hand - no need to use anything for leverage. I guess I was lucky. If the hole wasn't so deep I probably could have used my finger to take it out. When I was taking the plug out to check it that second time, I did have some difficulty getting the plug remover tool straight over the plug so it may be my fault that I snapped it off, but what I'm wondering about is the color of the insulator. If you look in the picture, the spark plug in the middle is the one that broke off inside the head. The insulator (which is normally covered by the threaded part) is black from carbon. The plug on the bottom, which I put in a vice and hacksawed the threads off, has a clean white insulator. This plug has been run for 40 hours or more yet the insulator is cleaner than the one above it which has only been run for an hour or two. Another thing I noticed is that all the old plugs I have have a nice coating of carbon on the plug that can be scraped off with a fingernail. I looked down into the engine and noticed the top of my piston is also coated with carbon and it doesn't look clean. How much is too much? Is it normal for the plugs to be this dirty in a 426? NGK's website doesn't seem to agree. All the old plugs I have look like the second picture "DRY FOULING." I know spark plugs can't be 'read' in the same sense as they can be when burning leaded gasoline, but it does seem like a lot of carbon buildup on my plugs. Could this be jetting related?