In what position is the engine mounted? It the engine is upright and the middle of the gas tank is higher than the carb needle, you could be flooding the engine because fuel is dripping into the carb before you start it. That would explain why you can start it after closing the high speed needle. If the gas tank level is set up right, the high speed needle will have no affect on starting the engine.
I like to mount my engines upright and I use a prime bottle so that I can put a few drops of fuel into the carb to start. I choke the carb only to see that the fuel is brought up to the carb. My engines hand start with a couple of flips with this method.
Sorry, I did not notice at first that you are using an Iron Bay regulator. I have no experience with that regulator. Try disconnecting it and using the above method.