my new bike did that, too. it ran great at half choke, died at full choke, and sputtered at choke off.
i checked for air leaks and found none, then thought about it, and realized that an air leak should have nothing to do with it. it would need a massive air leak to make it act like a choke, and since it dies at full choke, i know it's not sucking any in anywhere else.
so, knowing that the choke blocks off air to make the fuel richer, i lowered the needle clip to the third notch, and rode it all day long with no problems. and my plug looks good. not great, but about as good as it's gonna get.
(i initially had the clip on the top notch, because of my Opti2 problems, it was the only way it ran semi-good.)