I got a junked express that was doing much the same thing. The only way that I managed to get it running was to play with the choke a LOT.
I had the bike up on it's center stand and it would occasionally start when choked, but when it did, it would not idle and rev right on up. If I backed off the choke, it would die, but gradualy it became less and less sensative. My guess is that it was just cleaning out its carb, so be persistent.
Not that any of this matters now that the starter spring is broken...