Thanks again. I found the address for shop from their website and visited it yesterday (4 August). Mr Asif (the owner and I think moak89's father) was present there along with their mechanic Amjad. They did a compression test and declared the result satisfactory. All four cylinders were showing around 210 (bars, I think:S). Next they checked the electrical cables and plugs. All were ok. Finally they tuned the engine for CNG and set RPM around 1100. Mr Asif also recommended servicing the carburettor as I have never had it serviced. He suggested Mahmood in Plaza or Rehman near Tariq Road. For me Tariq Road is more convenient so I might go there.
After visiting The Tweak Shop, the car's idle has become smoother and on the drive back I noticed a little missing once I was coming down a slope in 4th gear and not pressing the accelarator. If I let the car coast without pressing the accelarator it does a little missing.
My next step would be to have the carburettor serviced. I am just waiting for the time to be available to me.
Any comments or suggestions would be most welcome.