I own a 1997 XR 400. I was recently riding and it started to run rough. It sounded like my plug was fouled so I changed it. That did not solve the problem. I decided to cut my day short and spend some time the the bike in the shop. After it had cooled down, I tried to fire it up again. It still ran choppy and I noticed that one of my header pipes was red hot after a min or so. I tore the head off and nothing fell out or appeared broken. I noticed the one of the ex valves was perfectly clean where the others were carbonized. What is all this meaning? Does anyone have an idea about my problem.