I have the same problem with my mehran
runs fine on cng but like something is holding it when it is on petrol, first it kind of slows and then with a jerk it picks up which again slows at the higher Revs in that particular gear.
Got the car checked through several mechanics including Suzuki 3S, they said we need to change its VENTURI to a genuine one, which will cost PKR 700 for mehran. another mechanic said we can get the holes in venturi widened for air mixture of petrol through a CNG expert.
Despite of these several solutions, i have not got any solution done till date as genuine VENTURI back then was not available at Suzuki 3S and i did not opted for other solution and was too lazy to check again on 3S.