The long engine configs suffer from this problem. It’s that the fuel gets to travel a long way from the carby to the extreme cylinders, and (that part is my wild guess, since everything I know about engines comes from this game, and the game does not explain it) it affects the fuel mixing, causing a less consistent and less precise fuel mixture.
What I’m completely certain of (in terms of game mechanics at least) is that If you put more carbs onto it, it should be way better.