Difference? They are completely different. The needles and main jets *may* be interchangeable, but everything else is different. I would bet that the 28 carbs are *easier* to overbore since the bore is round, while the 21 carbs are an offset kind of oval bore. Even then, the needle-jet on the 28 is non-removeable so you cant get a boring device to go through without hitting it.
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Paul Herr
'88 FZR4/GSXR/YZF Frankenbike
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