You need to make sure the linkage is level with the ball on the carb. If the ball is higher or lower than the linkage at the servo horn. It will wear and start to bind. It needs to pull perfectly straight. Some vehicle designs you will need to raise or lower the linkage off the horn, it also depends on the angle of the ball on the carb. I saw this happen to a guy at the track sunday who was running a picco engine. He had his linkage higher than the carb ball and it was forcing the slide up into the body everytime he pulled the throttle. Then it eventually just stuck. I gave the guy a old P7R carb I had in my box. and set it up for him. I think this gets overlooked a lot.