I ran a pair of mc21 carbs on an mc28, it didn't go notably better or worse than on '28 carbs.

Re. the HRC jet kit yes, the HRC jet kit has the power jets blanked off, bigger mains and adjustable needles. Once it's set up you'd struggle to tell a bike with the PJ working from one with them blanked though adjustable needles are nice if you need to adjust the needle postition.

If you're running the stock smartcard it's never going to set the world on fire.
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