Very good point you made with it priming cold.
This is going to be a process of elimination, best to start with the less in depth things even if they are less likely.
Yup that's the exhaust I thought. Some diameter fuel pipe doesn't fit snug enough in the hole causing a lack of pressure in the tank. Check it's snug.
Take the lines off and blow them through or replace if you have spares, clean any fuel filters.
Take the tank out, leave the tubing on. Submerge in water, block off one line, blow down the other. Does the filler cap leak? Or any other part of the tank for that matter.
Take the silicone joiner off, inspect for splits, refit with new cable ties, nice and tight. That's a tight angle it's at.
Check all your needle settings. Do the carb gap first then set your LSN HSN to factory (will need to tune later) do this with the servo linkage disconnected and the carb rotated shut.
If you have carb imbalance and the LSN is super lean it will struggle to get fuel past the carb.
Reset the throttle servo horn and linkage to carb (trim on TX zero'ed before hand)
Failing all that, check cooling head bolts are tight, remove carb and back plate and re-seal with RTV/Instant gasket. Would be worth doing the manifold too.