1996 Hyundai Ls, Enormous Fuel Consumption Problem !

HiI'm from Europe from Macedonia. I have 1996 Hyundai LS model and I don't even run it,because the fuel consumption is enormous. According to the specs,it should spend about 5l on 100km.It was everything OK until I took it to the first servicing.I don't know what did mechanic do,but after that it started spending about 8/9l and then because of that I took it back again and after that the fuel consumption is even higher.About 13 litres now! I took it to a Hyundai official mechanics and they couldn't do anything to repair it and make fuel consumption normal.Because of that I rarely even drive the car. Can you please please help me,what should I do,should I change the carburetor or what? What is the problem? Thank you in advance very much