Let's assume you put the engine back together properly. 1)Check fuel lines, tank and muffler. Make sure there are no obstructions and that they are connected properly. If, for example, you left a plug in the muffler exhaust pipe you would get the symptoms you describe. 2)Check your ignitor...is it lighting the glow plug bright orange?

OK. On to more drastic measures...

3)Disconnect fuel line and shoot some starter fluid (ether) into the carb....then try to fire it. It should start popping. If not, take it apart and rebuild it again, something is wrong. If yes, connect fuel line, bring the fuel up to the carb, but do not prime. Give it another spray of starter fluid and fire it up. If it floods, check your needles, especially the low end.

If you still can't get it to start, get someone to help you.