On the side of the choke stove you'll find a hole with a common screw head inside and, possibly, a nut on the opposite side. This is used to adjust the spring tension that closes the choke door. Tighten the adjustment until the choke closes on its own and there is noticeable spring back when closed.

Also, be sure the choke pull-off diaphragm is working or the added tension will prevent the choke from opening on startup.