I had the same problem in mid of july. The back fire never stopped. So called a special person on this forum Spectra the CNG specialist and did the following as asdvised.
Changed the Plug wires. got kabli ones for Rs 1500 bilal gunjh.
Changed the Plugs 4 tipped BOSCH FR 78X Rs 1350. Montgomery road.
Changed the air filter Rs 280 fm filter house Near Adil Hospital DHA.
Serviced the Trottle body (Self). Can be servised for Rs 500-600.
All cost about Rs 3100. The Car is ok now.
Pls keep the air filter clean every week and change it with the oil.
Increase the gas intake also specially in summer
If coils need to be chaged for sure try chinse ones they range for Rs 6000 (3000x2)
The Kabli ciols cost Rs 9000-10000 but its a luck/risk
Your Elec Generator must be in good working condition and shd be used genuine one and no other is to be tied in as if it is changed, will for sure change the current ratio to all the electric parts especially the electric coils. (This is experienced pewrsonally ).