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Now, I'm super excited to try your next suggested trouble shoot, I'll clean the filter and see if it makes difference. Further I'll try tank vent test, easy to complete. And then I'll figure out inspection the fuel port filter. But the vacuum thing you said, did not make sense to me, I didn't understand a word, but let's see how I go with step 1&2 and then I'll worry about next...
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Vacuum is opposite of pressure, less than normal air. You use bicycle pump to add extra air and pressure to tyre. A vacuum pump as shown in photo sucks air out. Some people suck on small hose attached to petcock's vacuum diaphragm to activate it and cause fuel flow. But that doesn't give you quantitative measured number of how much vacuum is needed to turn on petrol flow.
With worn-out or leaking petcock diaphragm, it may be possible to make it flow by sucking on hose, but engine may not generate as much vacuum and you won't get enough flow. The vacuum activation hose is smaller one to right/behind main fuel hose coming out of petcock in your photo.
In wonder if this is OEM petcock? For U.S. bikes, petcock doesn't have lever. Just slot for screwdriver to turn position. When doing vacuum activation & fuel-flow test, petcock should be set to normal ON position.