fuel mileage does 200km with 30 litres of fuel

This is not great, however, if you have a heavy foot then this would be about right.

What I would suggest doing is not messing with the carb set up yet. If the vehicle is running well and not sluggish or smoking, then it is most likely not a fuel issue.

I would suggest with doing a traditional tune up and here is a guide that will help with this.

https://www.2carpros.com/articles/how-to-tune-up-a-car-engine

https://www.2carpros.com/articles/how-to-replace-a-car-air-filter

If the engine is running poorly then this could be a fuel related issue. However, I would suggest starting with the tune up and air filter and then recheck the fuel mileage.

Poor fuel mileage is more likely to be related to poor spark so let's start with this and go from there.

Thanks