Monkey boi 89

I took my CG to the local motorbike shop and they're saying my bike needs new points putting in so going to give that a go and see if that works thanks all

I have a problem with my CG 125 it keeps backfiring and not running properly so I took the spark plug out and it was black like soot so I change the spark plug and it runs it run fine for five minutes and it's doing exactly the same again I took the spark plug out again and it's black like soot its was a brand new spark plug I do not know what the problem is can you help me please thank you