1) clean machine top to bottom of dirt, lint, etc.inside & outside.take case apart, oil & lube where necessary.
2) insert needle with groove facing bobbin. slowly turn hand wheel .. when the needle reaches the lowest point of its travel in the under part where the bobbin goes, look at the hook ( goes round in circles surrounding the bobbin, has a pointed ''arm'' on it..that pointed arm is the hook. the hook should be just next to the needle at this point (needle low point) & as the needle begins to pull up, the hook should pass the needle (here is where it grabs the upper thread & twists it around the bobbin thread. If it does this, the machine is timed. If it does not, then loosen the set screw on the shaft that holds the bobbin/hook .causes the hook to rotate) , loosen & adjust hook to do as describe above.retighten & test. The hook should just touch the needle when it does this.
Now, examine the upper thread path..the thread MUST pass through the tensioner leaves or tension knob with plates in it.as u screw down the tension, the thread will become more taunt.usually a setting mid point is correct in the stitch tension range. Now, with the machine threaded on the upper part (and through the needle) pull thread..it should move very easily. NOW, lower presser foot & the thread should have become hard to pull (the tension leaves r working. IF it does NOT do this, the release on the pressure leaves is frozen up.oil with WD 40 & work until the tension leaves r free moving & apply tension to upper thread when foot is down.
(usually, a gnarled thread mess is from lack of tension on upper thread).
Wind bobbin, examine that it is even, lots of tread on it. Place in bobbin case so that thread passes (probably counter clock wise movement when thread is pulled off bobbin in its case.a few machines work in reverse..like Viking, but it is very rare. All Japanese & American heads rotate counter clockwise). Make sure the bobbin pressure leaf is free of lint under it, is smooth & allows thread to pass easily. Now adjust screw to place some tension on bobbin thread.does not take much. Lack of bobbin tension will give a lousy tread on the bottom of the sewing line, but will not result in gnarled thread. Adjust the tension so that sewing stitch twist ends up in middle of fabric being sewn, not on top or bottom. This is achieve by balancing top & bottom tension. The machine should work if u have done all this.
Check upper threading..from spool of thread, the thread should pass through some kind of guide, go down or through the thread tension plates (a dial perhaps, or leaves of metal), then through the eye of the upper thread looper (the part that goes up & down when the machine turns..this pulls the tread enough to allow the stitch to work., through guides to the needle, through the needle on the groved side & then under the pressure plate. With bobbin in place & holding the needle thread, turn hand wheel on revolution , then pull thread & the bobbin thread should come up through throat plate. Place BOTH thread to the rear of the pressure plate. While u r at it, examine the plate the needle goes through that it is free of rough snaggers around the needle hole. If it has been damaged, sand & polish until there r no hang up places for thread or fabric to get caught. The plate can be damaged if the needle glances & hits the plate when the machine is used. ALWAYS MAKE SURE THE PRESSURE FOOT IS DOWN BEFORE YOU START TO SEW.Otherwise, there will be no tension on the upper thread! good luck