Hello Laura,
As far as your Kenmore 10 is concerned it is quite possible that the teeth may be a bit sharp on the edges, not a lot you can do about that, remember that sewing knitted fabrics is not easy anyway, I would suggest that sewing knitted fabrics, suede or leather or any silky fabrics that you use a toilet roll on your lap and feed the toilet roll under the fabric when sewing, can be a bit of a nuisance pulling out odd bits but it does work.
Your Kenmore 148, loosen the stop motion as if to wind a bobbin, put the foot control down gradually and see if it picks up speed, if it does then it is probably congealed oil on the machine over the years that is slowing it down in which case get some WD40 and and oil all moving parts, if the motor just runs slow I would check the foot control first, on the old foot control the resister was a coil of wire wrapped around a block and when you pressed on the control a small wheel moved up the block, what used to happen was the coil of wire used to wear away and break, unplug the machine and take the top off the foot control and check, if the wire is broken it will need a new foot control, any make of control will do, you can fit all makes, you have to decide whether it is worth spending the money on a new control.
Any problems you can contact me drivewithken@blueyonder.co.uk
Ken
drivewithken@blueyonder.co.uk, October 2007