Q: Replace carb on my 79 Cadillac. I thought it came preset. The car cranks but only runs a few seconds than cuts off.

Rebuilt carburetors do, to some extent, come adjusted but the air fuel ratio and idle speed can be more finely tuned to the individual engine. In your circumstance, maybe the adjusting screws vibrated out of position during that first day of operation? Is the gas fresh? Before adjusting anything, I would first diagnose the stalling, that is make sure that the problem is really "fuel system" related. You can test fuel pump output and pressure to see if those are in an acceptable range. Of course, check for a clogged fuel filter as well. The stalling you are describing probably stems from a fuel and/or ignition system fault (check to see if the timing advances normally) although air induction could be an issue including the possibility of a large vacuum leak from, for example, a leaking brake booster (if equipped) and/or the PCV system. If you want this stalling diagnosed by a certified Mechanic, dispatched by YourMechanic right to your location, please request an engine stalling diagnostic and the responding certified mechanic will get this resolved for you. If you have further questions or concerns, do not hesitate to re-contact YourMechanic as we are always here to help you.