Sounds like it has an air restriction problem and/or it may be flooding. Make sure your air filter isn't clogged with fuel or other gunk, or simply remove it for troubleshooting purposes. (Don't forget to put it back on later!)

If that doesn't fix your problem, you might try a new glow plug. If that doesn't work, try leaning up the mixture 1/8th turn at a time, or simply reset your needle valve to the factory settings.

Just keep at it and you'll eventually get it running.