Don't Wait Till It's Too Late
Sewing machines should be serviced once every two years. Daily sewers, should service their machines at least once a year. If a machine is sitting for a long period of time (2 years or more) it should be serviced before re-initiating use. The oil in the machine will dry out if not used, and needs to be cleaned out and replaced.
A full maintenance service of your machine is like a tune-up on your car. When the lubrication of your machine is exposed to the air, it will begin to lose its value, creating gummy and hard deposits. The machine's parts then slow down, and can also seize. Internal cleaning and lubricating is a big part of your machine being serviced.
In addition to lubricant, all the mechanical settings are checked and adjusted. This includes needle position, the swing of the bar, needle to hook timing and clearance, feed dog height, timing, slack and binds in the movement of the shafts, removal of any burr's in the threads path, belt's tightness or looseness, and the balancing of the upper and lower tensions. After all of this is complete, your machine will be sparkling clean inside and a test sew is done.