When you say linkage do you mean carb linkage or pedal linkage? Pedal linkage may need lubed and/or return springs renewed.(carb return spring and any others). You should be able to repeat the sticking with engine off by having someone push on pedal while you watch to see if throttle returns to carb stop screw. I am guessing pedal linkage as in: Some hole is wallowed out or cotter pin is goofed up or just needs oiled. You can test your carb throttle linkage/butterfly shaft with the pedal linkage removed to see if it "snaps" back like it should. Then you will know if it's that or pedal or, as Bill says, the choke doing something.(engine off tests).

Right is right, even if everyone is against it, and wrong is wrong, even if everyone is for it. - William Penn