Weird Carb Behavior

Hello everyone,
I am driving a suzuki swift 1991 1.0L carburetor and its fuel consumption is really high at around 10km/L. The spec calls for about 17km/L.
I had the carb tuned by a mechanic lately but it doesnt improve the FC at all. I have changed to a new air filter also.
Cold start RPM is very low at around 600 and sometimes hard to start. I thought coldstart RPM should be higher than warmed up RPM. Warm up at round 1K. With Air Conditioning on, it idles at 1200RPM. But if i drive, engaged the gear etc, for a few mins, the idle RPM shots up to 1400RPM.
The cold start low RPM cause by the choke? How do I adjust the choke? anyone can help on where the Air/Fuel mixture screw is? Sometimes when i off the ignition, the engine was still combusting for 1-2 seconds!!! No kidding. Why? Help please ...

Thanks in advance.