I"ve got a 3647 model Century, and when it was brand new the shuttle was kind of tight or sticky. However, when I did the 50 hour service, part of that was greasing every zerk I could find. I must have greased something right because it immediately became much easier. You might look around for a hidden zerk somewhere. I remember there were a surprising number of them in hidden, out of the way places.
hope that helps.
anthony
