A couple of things to check.

Make sure you do not have a fuel system leak. Disconnect engine line and muffler pressure line. Block one with finger and blow into the other.

Check that one of your helpers has not completely closed the Air Bleed screw. There is a little hole in the front of the carb and a screw that partially blocks the hole. If this is completely blocked, back it out until the screw only blocks about half of the hole. Instructions for setting the air bleed are in your manual.

If the engine has never been run and is extremely tight at TDC then it can be very hard to keep running. My 40 LA was like this. Try heating the head of the engine with a heat gun before starting. You may have to run 3-4 tanks of fuel before the tolerances between the piston and the cylinder are right for cold starting. Search RCU for some threads on breaking in ABC and ABN engines. The proper break-in procedure will cause the tolerances to change without excessively wearing the piston and cylinder surfaces.

Carl