It's not an unusual problem for the 401, I guess the 501 suffers from it as well. What's happened is that the old dried oil has turned to varnish and is gumming up the machine so that not everything moves at it should. It takes removing that old oil and replacing it with new oil. You can clean it with several products like kerosene, alcohol or just more oil. Sometimes you have to resort to heat (hairdryer) to melt the old stuff and get it moving again. It's not hard to do, but sometimes you have to clean and oil several times before the machine is totally free. (and that thing that sticks out in the back when you push the knob is normal)
Several detailed posts on this problem have been posted previously. I couldn't find them with a quick search. I have an appt., but will come back and find them later today, if some else hasn't done so already.
bkay