ORIGINAL: P-40 DRIVER
You may simply have the engine to lean to run after it runs out of prime. You may have dirt in the carb inlet which is causing it to go lean. Are you using a fuel filter?, if not , get one. Do you see bubles in your fuel line when running?, if so you have an airleak somewhere. Are your fuel lines reversed. Is this a new engine?, if so are you breaking it in per instructions. Did you try replacing the glow plug?
It is a new engine. I have ran it with the HS needle everywhere from 2 to 4 turns out and it runs best at about 2 1/2 out. I have started it at half throttle and ran it wide open and it will just all at once quit after about 20 sec. even at wide open or any other throttle position. I pulled the carb off and set the low speed needle so then end of the needle is right in the middle of the "cat eye" just like the manufacturer suggested and have even turned it out a couple turns. And i had the tank out last night and i am positive the lines are hooked up right. The vent goes into the tank then curves up to the top.