My nissan sunny 98 twin cam 16volve engine is working fine on cng but n petrol its little jerky and also seems like not burning fuel completely. as a result fuel average is bad. Because its throughing petrol like unburned and smelly exaust. Plugs, tuning, wires, distributor everything changed. What could be other reason. I need some intelligent mechanic or electrician who can solve this issue. Becuase this unburned fuel is effecting my fuel average and car pickup on petrol too. Also if anyone can give some good idea.

Im also thinking about engine swap with best fuel average and power plus parts availability.