I was fiddling with my fairly new-to-me green 319W and it seemed to be running fine, but then all of a sudden it tightened up to the point that the motor would no longer operate the machine. I wondered if a piece of thread had bunged up the works but on inspection I didn't find anything. I ascertained that the lower main shaft (Singer terminology: "rotating hook driving shaft") was somehow the culprit, and from there I narrowed it down to the shaft going into the hook gearbox on the left end. At least, I hoped that was the problem because it was a major endeavor to pull the gearbox.
... off it came. To get the gearbox off, several other items needed to be pulled off first, as can be seen above. Once off, it was clear that the shaft running through the gearbox was all gummed up. Some light cleaning, degreasing, and re-lubing got things moving along fine.
Now comes the fun part of getting it all back together and timed properly. After installing the bare gearbox, I found out how the two gear set screws went back in, as one was longer than the other, which essentially keyed the gear to the shaft in only one position. I then punch-marked the end of the gear as to where the screw holes were, as they are hidden from view once the gear is slid back onto the shaft.
Access to the two gear set screws is through a hole in the bottom of the main gearbox, and it is assemble-by-feel. The hole gets plugged with a screw once the set screws are tight.
The gear is back on and greased up.
Now for timing. The photo below shows the timing mark on the top end of the needle bar. The hand wheel is turned until the needle bar is at the bottom of its stroke, and the top mark should line up with the reference pointer.
Slowly rotate the hand wheel further and line up the bottom timing mark with the reference pointer. This is where the hook point should be directly behind the needle.
It's a little difficult to see here, but the hook point is directly behind the needle. Now tighten the two set screws on the hook.
The 319 has a bobbin case holder locating finger thingy (Singer parts manual calls it a "rotating hook bobbin case holder position bracket") that now needs to be installed and adjusted so that the upper thread can easily pass through the gap between the finger and the holder but still keep the holder in position. Singer says this gap should be 0.020 +/- 0.002 inches. It was late so I eyeballed it.
Here is a screen shot of Singer's adjustment procedure.
The sewing machine is all back together and working well.