finally i was able to take the vehicle to a mechanic. he disconnected CNG, found that butterfly was cut, airfilter was not present, timing was out, tappet setting was out.
after doing EFI tunning, repair of butterfly and installation of airfilter the car was giving following response
engine sound was reduced
i was feeling that vehicle is much lighter
engine pick was increased
knocking sound was significantly reduced
mistree told me that car engine is weak, needs overhauling. however i am feeling that engine still can go. i have checked for enigne replacement as well, which will cost me 50,000 -5AFE engine price and another 50,000 loss when i will sell it as people don't trust replaced engines.