IMHO... Not one of my various engines have a gasket on the backhead ... so I don't think that's an issue.

However, one thing that hasn't been mentioned yet (though it has been alluded to) is that sometimes the pressure tap on the muffler can get clogged w/ old fuel residue thus preventing the exhaust from pressurizing the tank. Blow through the pressure tap to make sure it is clear, and if there is a restriction just run a wire or small drill bit through the tap to clear it. This would explain your original condition (running briefly w/ a primed carbeurator then dieing). You can also prime the carb by blowing through the fuel tube that attaches to the muffler (pressurizing the fuel system by 'mouth') and see if fuel is reaching the carb. This also fills all the fuel tubes for that "first start".

Your initial complaint said that the motor would only fire at about half throttle(?) ... this would suggest that the low end needle is remaining closed until you advance the throttle. One way to set the initial low end needle setting is to position the carb at an idle position (perhaps 3/32 inch opening), blowing through the fuel line that attaches to the carb and listening for air escaping into the carb body. Close the low end needle, then while blowing, open it until you begin to hear air hissing in the carb body. Close the high end needle, then open 2-3 turns. If you have fuel flow and a functioning glow plug, the sucker should at least start. When I've fiddled w/ the needles and nothing is working, this is how I return to a base line starting point when tuning a motor. . Good luck ...

Live long, and prosper
Jerry L. Gubka