@Tayab, that problem is very common in CNG Altos, also was in mine, the problem is in cng cutoff switch solonoid, when u start the car starter sucks current & solonoid not open properly, install a relay on cng solonoid, for this, a guy name Afzal at byco petrol pump opp Honda point bhutta chowk, loose ur cars petrol cap once in a week also it will reduce pressure of gas in tank cause petrol can return back in carburater when running on cng.