ester,
To review:
>The 401 does not use belts. The motor uses a direct gear drive to run the machine. This motor does not have oil holes and is not normally oiled. If the top bearing howls you can put a couple drops of 30 wt motor oil down on top of the bearing and it will soften the grease in the bearing and stop the howl.
If it smokes it is probably dirty or contaminated by excess oil poured down on the gears. The gears require grease, not oil.
>The 401 is an oil hog. Many, many, many places to oil and some to grease. Because of this they tend to gum up if they haven't been used in a long time, or someone used the wrong oil as Miriam said.
>The 401 runs better when it's clean under the hood and properly lubricated. The difference between dirty and covered with old oil and clean and properly lubed is astounding.
>The 401 has certain places that if the old oil gums up it will bring the cam selection, or the ZZ to a screeching halt.
>The 401 is not a machine for someone afraid to service their own machines.
>The 401 is not a machine suitable for a novice sewist.
>The 401 is a PITA to learn if you don't have the owners manual. It's the first thing a person new to the 401 needs to get.
>The 401 has been covered from top to bottom in many threads here as Miriam has said.
>The 401 really loves TRI-FLOW oil and grease. I strongly urge you to get some of each. I never believed in specialty oils until I tried the TRI-FLOW products. I am a believer now.
>The same things can be said about the 500 and some other Singer machines.
Joe