Hi, I own Chevrolet Optra from last 1 yr. I have fitted CNG kit around 3 mnths back. initially it gave me a mileage of arnd 17 km/kg without any issues in pick up. However now i am facing problems with pick up. i took it to a mechanic & he asked me to change the spark plugs/cables. I replaced it. But still its giving me problem in pick up & mileage. Also, car gives hiccups sometimes.is it due to the defect in spark plug cables?