Hey i went to the dealer and he fixed it!!!
The reason was that the wire connecting the front wheel to the tachometer was a little shorter than the isolator covering it.
That explains why the tachometer was only working when crossing bumps on road.
But i got no extra for it but my bike ran further than it shows on the mileage counter! 0_o
Also i encountered some riders who had the same fault including fuel meter not working when riding a brand new ybr 125.