There were two HRC kits for the MC21, an "unleaded" kit and a "race fuel" kit.

If you are successful in finding an unleaded kit, I believe the MJ sizes range from 160 to 195, but these kits aren't easy to find. The more common (i.e. readily available) Jha kits are designed for 105 octane "premixed" fuel and range from 180 to 210.

Individual main jets can be ordered from ANY Honda main dealer for a couple of Dollars each. The formula for the MJ part numbers is on the Carburetion page of the 250 Tuning section.

Andy.
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