The '18 barrels will help as their porting is better than a '21's. However the cases and crank are also better in a stock '18 over a stock '21 so you will never fully convert one.

Red, stock MC21 with stock pipes, moded air box and wire splice.

Blue, stock MC18 with stock pipes, moded air box and wire splice.


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If I have to take the carbs off once more...