Dont think you would need an advancer in an older non-EFi Cultus.
If your car cannot go above 120km on petrol without A/C even, something else is wrong.
When was the last time you did preventive service and maintenance ? Carb cleaning, plugs change, plug wires check (and change if necessary), dizzy cap clean, CNG kit service etc. ?
Mind you however, I wouldnt really suggest going over 100-120km with 5 people to begin with. Thora khuda ka khauf karo.