New engine starting issues

"Unwire" the ignition system AKA isolate it from the harness. WHAT IGNITION system are you running? Hot wire it to eliminate various harness/ connector/ switch problems

You might consider eliminating the fuel system by plugging it, and running a hose from a siphon/ gravity feed container above the carb direct to the carb fuel inlet

It may be a timing/ carb/ throttle position issue, AKA the throttle blades may be out of the transfer slots. Where is the timing set? If you have a "hot" cam and the timing is less than say, 15* initial --you need to bump it up

Crank the idle screws out some at least 'til it's warmed

Check for vacuum leaks