for a few weeks my brother crf 100 wouldnt start. so today i went down to the shop. and bought a spark plug. so i put it in and rode it for about 5 mins when i toke it out it was black, when i was riding it would back fire a couple times. i warmed it up for couple mins and rode it around. within 5 mins it was dark. what should i do?. whats wrong with it. its been ridin 10 hours. toatal, and its 04 model.