I am getting terrible gas mileage after overhauling engine

Hi, I have a 1995 Lancer GLX 1300cc 4 cylinder auto transmission. I bought it 3 months ago and when I bought it the engine was week so I got it overhauled properly. Even before the engine was bad it gave a very poor average that would be 4 km/lit max. After the running and tappets adjustment still its giving the same average, all sorts of adjustments and tuning did not help the average. When it was diagnosed on CO2 diagnoser computer it was emitted 20% oxygen.I was told to change the carborater so I did. It has a piston carb. Have done every thing to it. Nothing is wrong in it at all drived smoothly just just the BAD Average.I sick of spending on it because I have almost change every thing related to the engine. Engine compression is alright.I don't wish to sell this car as I did alot of efforts to it. Please advice me what to do. Any low cost solution for it as every thing is running perfectly. It just eats fuel as if its some 8 cylinder engine. It should aleast give me an average of 12 per liter.