Well, I've heard that they kinda suck and some people just tighten them right up.
Or do what I do: Warm the truck up and hold it down firmly with the body off.
Then gun the throttle while watching the slipper and spur. When it's set the spur should hold the engine back while turning slowly. Be quick about it though, you don't wanna overheat your engine, engine clutch, or slipper clutch.