If its a crimped vacuum tube the bike will fire up and then die because the vacuum sucks the fuel back into the tank. Blocked tank breather pipes can also cause this, its the easiest thing to check.
So you mentioned that a brand new spark plug wasn't sparking and when you used easy start - nothing happened. Why are you still looking for a fuel problem. The problem is either in the LT or HT part of the ignition system.
I know but there seemed to be a lot who didn't pick up on it. The next time someone posts a similar fault, it might just point them in the right direction if they saw this post.
This sounds like wiring harness problem, there will be joined wires in the harness which are susceptible to corrosion.Do not assume a wire is continuous just because it goes in and out of the the same colour. Every bike I have worked on since the 80’s has this issue. You need to give yourself time to identify all the wiring that runs the ignition and finger trace them through from beginning to end, to be sure they are continuous. If you feel it is fuel when the plug is out and safely away from your face smell the air coming out of the plug hole when turning over. Does it smell of fuel?