Really need a bit more info.
How exactly does it die? Revs drop quickly and then it cuts, or rise sharply and then it dies? Will it stay running if you leave the glow igniter on the plug?
I'm guessing you're running it in, so it'll be pretty rich which is the most likely culprit. There are a couple of options depending on how you like to break in engines. You could leave the bottom end really fat and bring up the idle to compensate which is the old-school way of doing it, or alternatively lean the bottom end a small amount which is how the heat cycle guys go.
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