I own a Yamaha r15 which has done 19k kms, the problem is that my bike

I own a Yamaha r15 which has done 19k kms, the problem is that my bike exhaust emits black smoke from few days. I'm using motul full synthetic oil since 10k kms, which is change periodically. Mechanic said its normal for 20k Kms counting bike, he advised me to change valves. What must have been the real prob?

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