Yup as Bigedmustafa said, start by assuming that the whole fuel setup is bad or clogged.

That your engine runs for a few seconds tends to point to the carb, head, glow plug, etc. being OK, although your mixture may still not be correct.

An engine quitting after a few seconds can also be caused by insufficient tank pressurization....

Anyway, as he said, pull ALL of the tubing out along with the fuel tank.

Using a clean piece of tubing, hook one end to the High Speed needle inlet nipple and blow in the other end while moving the throttle control.

At full throttle you should be able to get a HISS out of the carb body as you blow.

Then check to see that the tubing and piping into and out of the fuel tank permit free flow in either direction.

Finally clean out the muffler pressure nipple. It may be clogged.