I am posting these instructions according to my experience with Cultus. Mehran should be similar.
1- Warm up the car. Wait for the cooling fan to kick in.
2- Ensure that all electrical appliances are off; wait for the cooling fan to turn off.
3- Behind the airfilter box on the carb is a white plastic screw. Rotate it clockwise or counter-clockwise to increase or decrease idle speed. If you don't have a tachometer (RPM meter), set it at the lowest RPM at which the engine will run smoothly.