If you are referring to the main mixture adjustment screw with the spring under it, which I believe you are, I would say the spring is weak or threads are bad. Pull the main and idle screws out and compare the springs.
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Travis
1993 Cub Cadet 2064
1988 Cub Cadet 2072
1980 IH Cub Cadet 782 w/CH20
1966 IH Cub Cadet 102 w/K301
1961 IH Cub Cadet O
1967 IH Cub Cadet 102 & 122
   
JD 2155 w/ 175 loader
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