It could still be the petcock valve, but did you check the vacuum line for holes, rips, tears, and a tight connection at the carb and petcock valve nipples?

Sounds like its not sucking.

I don’t have a stock carb and I don’t remember the exact vacuum line connections.  That said, you do have the big vacuum line routed to the petcock valve correctly.  ANY vacuum leak will cause performance problems.  Fix the loose vacuum line and report back.

That's probably it if it's happened before.  Have you tried cleaning the gas tank?

Dave published a great post about gas tank cleaning.  I can’t find it.  Maybe Dave will chime in and can share a link.

One of the steps is using dry walls screws shaken around in the tank to knock off loose rust.  I can’t remember if he uses gas or water in this step to shake around with the screws.  I use cleaning vinegar in this step.

When you pull the tank and petcock, look for evidence of rust (the filter screens on the petcock valve will be a stained rusty red color).  You gotta get any loose rust out of the tank or it will keep clogging up.

Here’s a pic of a nasty rusty mess I cleaned up on a different bike.  I doubt yours is this bad, but it gives you an idea.

Once you’ve thoroughly cleaned the gas tank, you may want to use a tank sealer like POR-15.  I’ve never used it, but I have used Ospho as my final step.

https://por15.com/products/3-step-stop-rust-system?utm_source=google&campaign_id
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I love your process, Dave!  Thanks for sharing!

Ok, silly question: Where do you guys dump your old gas? I called my Town Hall and checked with the local Firestone, but nobody knows. Be nice if there was a place.