Royal Enfield — mileage drop & high black smoke

Address:

Jet Blaze Motors Plot No : 131 & 132, N.H - 4, Vellai Gate, Angala Parameshwari Nagar, Kancheepuram - 631502, Kanchipuram, Tamil Nadu, 631502

I buy Thunderbird 350 BS4 bike on April 2017.
Still now 6100 km only drive
Before my last service my bike mileage is minimum 29km/liter in city
I done a bike paid service in kanchipuram branch (
Jet Blaze Motors
Plot No : 131 & 132, N.H - 4, Vellai Gate, Angala Parameshwari Nagar, Kancheepuram - 631502) on 25/05/2019 regarding mileage issue.

In Royal Enfield service centre service engineer clean a carburater and inspect all part.
All are good

After service my bike mileage is reduced to 17km /litre and black smoke also producing.

I go to service center several time but still not response.