Can anyone tell me what the difference is between the MC28 and MC21 Keihin's? I have three carbies that came with an MC21 project, two are the same and have two A's in the code and one has a B, guessing the B is MC28 from a search of the forum? The B carbs look cleaner, and I have a HRC jet kit so need to decide which carbs to use long term. Are the HRC kits different for each? Thanks in advance,