I am no expert but it is my understanding from the previous forum threads that the RSE reed valve was not designed to be used with an 18mm Mikuni carb..
The RSE reed valve has only a small area even when fully open which many have reported actually restricts the intake flow at high rpm, and a reed valve engine's increased intake duration means that, rather than a quick gasp like a P.P. engine, it takes a longer inhale more slowly so it really doesn't need or want a large diameter carb that would further slow air velocity over the venturi.
In short, the RSE reed valve renders the 18mm carb redundant.
I believe you might find that the stock carb's diameter is more appropriate for the RSE reed valve; or that a P. P. intake or perhaps the four petal OZ reed valve is better suited to your 18mm Mikuni carb.