I have a problem. Can someone help with an advice?

I have a Merceds Benz type 190 E produced in december 1989. I am starting to have problems.
When i went in a long trip with that car I put some fuel in it and after that in first kilometers i felt that the car started to loose power. I did 200 km and the problems were increasing. I pushed the gas pedal but the car couldnt take speed and was loosing power more. The speed of car couldnt be more than 60 km/hour. I put the third gear even the forth gear but there was not reaction from the car. It was going in more slow speed. It seemed that it has a problem with injectors. The motor of my car is a motor 2000 cm3 and works with benzine. It has simple carburator with mecanic injectors. Then i stoped to a mechanic and he changed the group of karburator. He changed the place where teh injectors were suported and. The car started to function normally but i noticed that one piston was not working. When the minimo of the car was at the lowest level I had to turn on several times the principal key to start the car. When the minimo was in higher levels I didnt have problems with the principal key to start the car but i am having problems with the economy of the car. It is using too much benzine when i go with the car. Also it seems that the car is working in two pistons. Can anyone tell me what might be the problem of my car? I dont know if i said everything correclty because i am not a good specialist in motorric terms but if somebody what to help me I can tell more particularities later.
Thank you in advance.
Altin