I own a Mehran 1999. It's giving less mileage on both Petrol and CNG.

On CNG - 120 kms on 7kg approx on a full tank
on Petrol - some 8.8 km/ltr.

I know it should give more than that. Any tips and suggestions on what can I do or what stuff do I need to get checked for fixing it?

(There is an issue with the carburetor. It misses when you suddenly push the accelerator but works fine when you accelerate smoothly).

Help sought!

Also, need recommendations on engine Oil. The engine is in good condition - water dropping. What viscosity and API to use?