brother dont worry, my frnd have 2007 mehran and was facing the same prob, just do few things and it will be fine, his car was giving him 190 Rs gas approx 85-90 km on cng without Ac in city,
Do set these things by any gud mechanic and u will be fine.
do check the spark plugs if its possible change them, set the proper distance to use on cng.
clean air filter, check how long the oil is run , if needed change it,
u must set ur tappids (might spell mistake) adjustment for cng, cos the suzuki guyz on first check up of ur car will screw the tappids adjustment. Do loose them by some gud mechanic, it will produce some noice on idle , but car will be so smooth that u will feel the difference, also the millage wil increase as well as the power, pick and top speed.
After these adjustments my frnd is getting now 140 km without AC in 190 Rs gas in city, the mehran is FF cng ,
Hope that will helps u.