I reread your post...the plastic black nut on the outside of the cable (the part that's behind to the choke ****) may need to be tightened. do it by hand, do not over tighten.
It is not the hex nut on the back side of the mount, that is to secure the cable to the mount. Tighten the round friction adjust on the outboard side of the mount just behind the pull ****. Usually only takes a few degrees turn to hold the choke out.
I also let the engine warm up before taking off...sometimes as much as 5+ minutes, depending on how cold it is outside.