I have swapped 4V carbs for 2V carbs. The (solid rod type) linkage for a 4V is longer that for a 2V carb. If you put a 2V carb where a 4V carb was, the solid rod style linkage wll always be pushing on the trans arm - keepiing the trans in kickdown mode. This would make upshifts late and hard.

If yours has a cable type linkage this might also apply somehow. At the trans end the arm doesn't move a whole lot. It only needs to hit it's stop when the carb is at wide open throttle.

Tracy