Hi I have a merc 280se 1984 model I can go out after weeks and it will start first turn of the key but when I go to put my foot down it goes to cut out and it’s sluggish on the highway getting up to speed i was told mixture is out does that sound right any help appreciated

In an earlier post you mentioned that the former owner had fiddled with what you described as a "hex headed screw". Do you mean a screw with an external hex, or a screw with a hex socket (internal hex)?
If it is the internal type, it is probably the main mixture screw. If you know, is the car equipped with a catalytic convertor and an O2 sensor?