Just picked up a 2000 400ex all stock last week and have done all the maintenance work on it. Been driving it around the house and after I stopped it, came back out to ride again, it won't start. Took the plug out, its black as can be. So I clean it up, put it back in, starts no problem. Drove it around for another 10 min and decided to check the plug again. Starting to get all black again. When its running, it runs great, idles no problem. But why the black plugs. I shouldn't need to rejet it since its all stock. What should I look for.