Have you verified that you still have spark when it shuts down? To make this check, have a spare spark plug handy when it stops. Immediately transfer the spark plug lead to the spare plug, press the plug base against the cylinder head, and pull the starter. See if there are sparks between the electrodes. Remember, you need to do this within just a very few seconds of it stopping.
When you've done that check, if you do have spark the next step will be to dismantle the carburetor and fuel tank again, so do the spark check first, and report back. Then we can talk about the next part.
