Help starting my c20

[Socalrider87]

Long story short i got a new Jet carb to replace my old warped Quadrajet. Once i installed it the truck will fire up and die in about 2 seconds. If i hold the throttle down a decent amount it will stay on but once i go lower or let go it just dies. I replaced the fuel pump as well and still the same. I screwed the adjustment needles in and back out to match the old carbs setting and still nothing. Can anyone give some advice on what to check or adjust? Like i said with the old carb it would run fine just leaked fuel all over lol but with the new one i can’t keep the damn thing running.

‘70 C20 Longhorn
Factory 402