If you haven't already found it, it's behind the oil pump in one of the holes. (Which one, I can't recall right now and no books available while sitting on a plane at the gate.) Poke something in the holes to discover which has a ball and spring in it. You won't be able to service it unless you plan to split the cases. Make sure it's springy. Spray some carb clener in the hole. While I'm thinking about it, it would be interesting to know if anyone has measured the force it takes to compress a new spring a particular amount, and how they did it.
Richard L.