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?