Singer 206, 306, 319 and 320 timing

There are loads of these around and a lot for sale, so one you've modified the bobbin case to use normal needles, you'll probably want to learn how to ensure the hook is timed. Why? Good question, because these models have a tendency to go off due to the screws holding the bobbin drive to its shaft not being tight enough or maybe you tried to sew something too heavy and it slipped slightly.
This never happens with straight sew models, or I haven't ever had to set hook timing on them but quite a few 319s I've bought have needed to be timed. The other thing that commonly causes it is some dim wit in the past has tried to adjust the hook timing thinking it will allow them to use 15x1 needles.

Here is the procedure to check and correct the timing:
Step 1: Remove the end plate (cover of the needle bar) and the needle plate (slide plate) and tilt the machine back. Needle should be in the central position. Remove the bobbin case.
Step 2: Move the hand wheel toward you until the needle has descended to its lowest point. Now you need to check the marks on the top of the needle bar:

At the very bottom, the lines on needle bar and machine should line up

 If they don't line up, either the needle height has also been changed or (more likely) the static mark on the machine has been moved. If the height is wrong, you'll find out in a few minutes, so assume it's the little plate that has moved. You can see there's a screw just under it. Undo it and move the plate so they align.
Step 3: Move the hand wheel while looking at the needle bar marks. Stop when the lower needle bar mark lines up.

Needle has started to rise a little

Now check the position of the needle

Hook is exactly behind the needle, and should almost be touching it.

If the hook is not in the right position, Loosen the two screws holding the bobbin drive in place slightly and move the bobbin drive until the hook is the right spot.

Loosen (do not remove) the screws holding the bobbin drive to its shaft.

Now tighten the screws back up and do the test again. If it's still perfect (it might have moved when you were tightening the screws) make sure the screws are quite tight and check the needle bar height. Put the needle in the left position and turn the hand wheel until the hook is just behind the needle. The hook should be just above the eye of the needle.

I think this procedure is pretty important to know, even if you only have one machine. If you can't get the tension just right on your swing needle machine you can be fairly sure the hook timing has wandered a little.