Carbed 5.3 won’t start please help!

[Rdwagoner]

Built from bare block, 5.3, LS series MSD box. The engine fires up with first key turn and runs for a second or less and shuts off. Fresh build and first time cranking was today. I have a ground from battery to block, firewall to block. MSD box is grounded to intake, alternator not wired up currently. Was wired from main post on back of alt to positive battery 10g wire. Jumped exciter wire to main alternator post. Didn’t make any difference. Cranks for a second and shuts off. Engine is a 2001 5.3 from a Tahoe. MSD box is set to Truck on the front dial. If I was not getting fire would it not crank up at all? I had a friend holder open the joke and give it a little throttle when I tried to crank and it shop a little bit of fire out of the top of the carburetor. Any help is greatly appreciated !