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shspvr - lol no, there's no rat nest in there, I looked around (out of curiosity) already,
Also, you mean to clean the filter and try a test ride with fresh filter or without one? I'm confused!
@Danno - thank you! Yes, the bike has some mods done by previous owner, there's Yoshi exhaust, non OEM air filter (as you guys mentioned, I wouldn't have known), the bike feels to be lowered (I'm being 5.6" can comfortably flat foot both sides, which I couldn't on my cbr250), tail tidy and other I may not know.
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.
Also, when I was doing my first trouble shoot, to get fuel into spray bottle, I removed the fuel hose (like arrow 1 in image below) and then turned valve (like arrow 2 in image below) to prime and fuel was pouring out, but when on "ON" position, it barely came out. Does that mean anything? I would try your next suggestions on Wednesday as that's my next day off, and I'll get back to you once done
PS: I rode to work today, and luckily the bike didn't trouble at all on the way, let's hope it takes me back home too XD,