It happens more than you think. Just because you have a plenum doesn’t mean you won’t have distribution issues. Some of them occur because the booster design is wrong. Sometimes it’s because of the position of the carb over the plenum, even on a single 4. There are issues caused by big overlap cams disrupting air movement in the plenum and also cause a rich mixture, which requires more heat to vaporize the fuel. I’ve seen several times where getting the fuel curve cleaned up also cleaned up distribution issues.
