Make sure your mechanic knows what he's doing, modding the airbox and changing the timing ('21 flywheel) is when you start to need a lot bigger jets. The HRC jet kit has massive (170 mains is as small as the '28 kit goes, 140 mains is std) jets but is designed to run without the powerjets. I'd say probably start at 160 mains if you're just fitting bigger main jets and retaining the power jets.

***DO NOT RIDE THE BIKE WITH MODDED AIRBOX, 21 FLYWHEEL AND STANDARD JETTING OR IT WILL SEIZE***
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