Intake recifyers (or stuffers) shape the incoming air / fuel mix which improves the burn.

Fitting a spacer under the reed stop raises it so the reeds can open further, which allows more charge in. Opening it too far stresses the reeds which can make them break. Not usually a problem as modern carbon reeds just get shredded without doing damage.

Mostly...
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MC21SP Plaything
BMW F800GS Bumblebee
Triumph 9551 Daytona Big boys toy
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