Looking back at the diagram that Sportster has submitted, where exactly is that check valve that's circled in blue located? And what's it's purpose, or happens if it fails?
That check valve is a small flapper located on the kidney-shaped metering plate (also called metering block). It's a thin piece of mylar plastic held with a single screw and if it's missing. bent or even too stiff (think aftermarket part thickness), wont' work properly. It keeps fuel in the fuel chamber from entering the large venturi at low speed, once vacuum across the large venturi develops fuel is pulled through it.
It's referred to as the high speed check valve and it's purpose is to keep fuel from dribbling from the large venturi at low speed (low throttle angles). If missing or bent, there will be poor control of idle mixture using the low speed mixture adjuster and flooding at low speed. If too stiff, high speed fuel can come in too late and lead to hesitation as throttle is applied.
Vacuum at large throttle angles pulls fuel through this check valve. All the high speed fuel must pass through this check valve on it's way to the large venturi.
It also stops air from entering from the high speed circuit in reverse direction and upsetting the vacuum signal to the diaphragm at low speed. If air leaks backward through this check valve, the fuel metering needle won't be actuated by the diaphragm consistently, low speed fuel control will be disturbed and inconsistent.
I recently had another brand carburetor with this check valve actually mounted inside the high speed venturi (valve was a tiny disk on seat configuration) and had some debris trapped inside the valve. The valve would close sufficiently but wouldn't open correctly, the result was lack of high speed fuel caused fierce hesitation as throttle was applied.
Maybe I could've simply said this check valve ensures vacuum signal to the fuel chamber diaphragm is transmitted through the low speed circuit only, during low speed operation.