i presume that the feul filter only works for the petrol fuel not for the CNG.then why abnormal behavior with CNG( i might be wrong).
i guess the spark plugs are to be blamed
coz at first i began to get poor milage only 60 kms in 200 rs of gas.
then the car started missing, lack of acceleration then stalling and hard starting.(the engine seems very much silent than normal )
any recomended spark plugs