1993 Suzuki Margalla Carburetor

I have a CNG fitted Margalla. The original carburetor stopped performing efficiently. The body near the throttle valve side had cracked, there were jolts when the accelerator was pressed at low speed and the mileage was around 6 to 7 km per liter. My mechanic had serviced the carburetor many times and finally suggested to replace it. I took his advice and bought another carburetor. The new carburetor for my car was no more available in Pakistan. I had to buy a second hand carburetor. The second hand carburetor came along with the engine, Japanese make. The carburetor is of a new model margalla.

My mechanics had inspected the carburetor at the time of purchase and said that it was in very good conditioned and sealed (non-repaired/non-serviced). I bought only the carburetor for $263. My mechanic suggested fitting the carburetor as it is without servicing or doing anything with it since it was sealed. He said we would open it up later.

The drive of my car has certainly improved with sharp and smooth acceleration. The mileage within city is between 10 to 11 km per liter. Recently I took it out of city for a 40 to 50 km drive and calculated the mileage to be roughly 13 km per liter (with AC).

What I want to know is what is the best mileage I can get with my car and the carburetor? Secondly my mechanic is now suggesting to bring in the car so that he can service the carburetor and replace it jets. Should I be taking this advice of his? Also with the previous carburetor, the car had a choke system which allowed for higher rpm in cold start and also another automatic choke for higher rpm in AC mode. I find these two features missing with this carburetor. Does the car always start with choke in the morning even in summers? Another question is about the idling rpm, they rise from the value they are set to, like the other day the mechanic set them to 900 on petrol and after I drove a little they increased to 1200, why is this so?

I have attached a few pictures of my car under the hood. Please tell me if you can, where can I find idling screw?