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I've done this to mine plus several sets I've rebuilt for others when the owner commented about the lever no staying in place.
A self-sealing meat roofing-screw has a bonded rubber seal.  Replace the tensioning-spring with two of these washers.  Rubber side IN.  Between the back of the lever and the carb.-stud  is a small steel washer. Replace this with a larger washer to create more contact against the back of the lever.  The hole size of the larger washer allows the threads (only)  of the shoulder-bolt to pass through.  If the tension still isn't adequate, a thin rubber washer can be placed between the two roofing washers to increase the overall thickness.  The compression of the rubber suffices for the factory spring.
This holds the choke lever where it's set to..