i have a 98 wr 400 and lately the spark plug has been fouling as of late...it used to be about 20 rides before a plug change, now its twice a ride.the bike will run great for about 10 laps then it will start to pop on decel, then hesitate on accel, then run like sh*t til i change the plug.I'm running 93 octane pump gas. the plug is totally black when i remove it...too rich??? should i adjust the feul screw? if so which way to turn ? in or out? the engine was totally rebuilt last year and the bike has performed perfect until now. Can any of you guys help me?