New engine starting issues

Finally got to the point where we are trying to start our new engine to break it in, it starts for a few seconds and then dies. Turns over strong, starts up again and dies. Everything is new, from carb to pan, timing appears to be dead on, no backfire or sputtering. Good spark, strong fuel flow, definitely good compression. Checked the cap to plug wiring routing and it’s correct, set engine to top dead center, rotor is pointing at #1 when it fires up at first it runs strong then acts as if it’s starving for fuel, but it’s getting good fuel flow. Unhooked the fuel line and ran it into a bottle and it’s definitely there. Good fuel spray from the carb into the intake. Just need some fresh input as to what to try as we don’t want to wipe out the cam cranking it over and over. Thanks in advance.