Many users complain about bad pick up of Mehran on CNG. Well here is a tip for mechanically inclined PW'ers.
Swapping in crank shaft and connecting rods form Khyber engine will elongate your stroke to 77mm and raise your compression ratio to 10.7:1 and displacement to 851cc. Crankshaft may require some machine work to fit properly and cam timings should be done again to reflect longer stroke.
Long stroke with matching cam timings will greatly improve your low end torque making your pick up stronger and yielding better CNG mileage.
Beware that after this modification, you should NOT run your engine on regular petrol, it will knock pretty bad and self destruct. It will require high octane petrol for long term running. Engine redline will reduce to 5000 rpm versus 5500 rpm original.
It took a hell of a time at sher shah to figure this out. Hope it was worth.