the thing missing from your did-that list is the fuel pump
these bikes have one because the bottom of the fuel tank is lower than the float bowls, so the bike would run with a full tank of gas, but not all the way to empty
pull the fuel line off the carbs, turn the ignition on, and the fuel pump should run, at least for a few seconds, and spray gasoline out the hose.
If the fuel pump is not working, then unhook the line from the petcock and make sure it letting gas flow when you turn it on, then connect that line directly to the carbs, and if the tank is full, the bike should run.
if the direct connect fuel line does not make the bike run correctly, then I would think the ignition is dropping out when you release the starter button for some odd reason. An inline spark detector would let you see if the plugs stop firing when you release the starter button.
Last edited by KCW; 07-12-2019 at 07:28 PM.