Thanks for the encouraging responses. I used the y piece to connect each carb to my one aux tank (not the main tank).
It fired up immediately (or after 5 seconds). It was running on choke without the airbox and a steady tickover…
Only thing i did find the the starter clutch is just whrring now so guess it’s either not engaging or stuck engaged. But i can deal with that later, at least i know its a runner.