First off, try setting the carburetor per the instructions at the top of this forum. It may be that the relationship between the high-speed and idle needles is off. Also, don't try to start the engine at full idle, but at 1/4 or 1/3. That will help it get running. If the carb's not set, you can have difficulties at low throttle settings. You can have a loose compression feel until the engine has some running time and the ring gets seated.

If all else fails, we'll need to see the engine. It's possible that something's physically wrong and it doesn't have enoug comprression due to a bad piston or ring. Low compression will keep it from running every time.