I have City 2000 Exis. From few days I noticed car is running very heavy specially when AC is on. Sometimes car shutoff (AC on) I think its due to low GAS pressure or RMP.
Today I visited CNG mechanic he tuned CNG setting and when I went on test drive with him noticed same issue occurs on slow speed (AC still on) and car starts knocking. Also noticed RMP go up around 2000 ~ 2500. But car run smoothly when AC is off and no RMP issue.
Then mechanic recommended me to installed Advancer (currently not installed) also assured me that after installation of Advancer I will not face this problem.