Modern Buddy

I am helping a friend whose husband past away and left her several scooters. I am trying to get them running so she can put them up for sale. One of the Stella 4T's has a 21mm Pinasco carb. The bike was running, but the choke cable pulled out of the brass plunger on the carb. It has a set screw in the side of the plunger, but I can not get the set screw tight enough against the cable to keep it from pulling out. When it pulled out the last time, my first thought was JB Weld...lol. I have been working on bikes for a long time and never saw a set screw arrangement like this. Anyone have any suggestions?

Thanks...

Check to see if the threads are stripped. Got a pic?

The threads are not stripped. It tightens down snuggly but not enough pressure to keep the cable from pulling out with normal pressure. Since I did not install this upgrade, it may be too small a cable. This would explain why it keeps pulling out. I have just ordered a new authorized cable...hopefully that will solve my problem.