Sir
CNG needs a near perfect CNG/AIR ratio, you have to either increase/decrease the CNG supply or to block some air.
Petrol performance will decrease by blocking the air intake, but that is a compromise you have to make.
Also thoroughly check the air filter, spark plug and timing, which are very crucial for CNG, also tell your mechanic to service the CNG kit for any oil.
If still not satisfied with performance.
Try the Enercon at the Kashmir road or Honda 2S, Saqib Autos at Malir, they are good at CNG.
Regards