That's probably the little clear flap on the metering block. You can inspect it by popping off the diaphragm cover. That block comes out with two phillips screws. The flap comes as part of a rebuild kit. Not sure of it's purpose.
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.
Sure. So while inspecting this tiny flapper piece (might call it a reed), it should be laying flat covering a hole in the metering block. If there's some chance it's been replaced with aftermarket it may have been stamped from mylar film that's thicker than Mikuni's OEM part. If bent or has debris under it holding it from sealing off the hole, it's unlikely to function properly. Generally the original flapper is in good shape and doesn't need to be replaced, I prefer leaving them untouched if they pass inspection and not suspected of previous shenanigans.
Same goes for the two circular mylar disk check valves in the fuel pump body, as long as they seal flat against their seats and the seats aren't pitted from corrosion, these mylar disks aren't cracked, bent or damaged in some way, I see no need to replace them.
Thank you Thank you the low side was cloge
When I sprayed brake cleaner in the jet hole
It was hardly coming out after a few seconds it popped and sprayed out of the 3 small holes full force. She revved up and came back down nice and clean.
Thanks again for all the help and support I was ready to pull the engine. I have a week long trip coming in two week and I was starting to sweat I wasn’t going to have it broken in in time.