1st Start stock 340 w/ New or Rebuilt Carb?

What carb?
Timing on first start is usually a bigger problem for most people. Crank in a bunch of timing. Like 30-40 degrees at 2000 rpm for break in. If it’s a Holley start with box stock jetting, set the idle mixture screws at one turn out and the set the curb idle screw so the transfer slots are square or a little more open. If it’s got a choke, disable it for break in. If you have the bowls off flip them upside down and set the floats level. If it’s not a Holley get a Holley. The last comment should get some response from the carter/thermoquad guys.