Hi,
I had exactly the same problem, my engine wouldn't fire after rebuilding the engine. It turned out to have been two things: First, the ignition was set wrong: to get it to start I disconnected the fuel line und turned the engine only on the starter (took some life out of my battery ;-) and set the ignition 8° BTDC, then I reconnected the fuel line. The second thing wrong was I was stupid enough to leave one of my IMS screws on the carb screwed right in. Set that 1 1/2 turns out and after a turn or two it started. Ran very roughly, but I could set it OK from then on. Before I had set the dizzy right on TDC, and it didn't even fire, that little can make a change for starting it from nothing. Once it runs a wrong timing won't immediatley stop it, but to start it needs the timing failry exact because the engine turns so slowly on the starter. Hope that was some help, Good luck!