Fuel average is determined primarily through the supply of air and fuel. Usually bad average is excess fuel supply. Map sensor, O2 sensor, fuel pressure regulator and leaking injectors are primary reasons. You should look up these sensors in the FSM (check a thread called Dewan Lancer Manuals). Download all files of the FSM in one folder and then open the Start Here file, move to the index and then search for all these things.
Our mechanics are not literate and cannot understand and fix these things. Hence, the onus is on you.