Ralph,
If you sandwich the gasket betwixt the insulator and carb base, the vacuum path is through the skinny space developed by the thickness of the gasket itself. The little notch keeps this path open to the bore of the intake. I think this is how it works.
To be honest, I never have understood why the fit of the insulator to manifold is OK without a gasket but the fit of insulator to carb base needs a gasket..makes no sense to me, but I'm just a dumb Millwright.
Stuart
