Check the posiion of the tucker finger drive cam lobe relative to the left knotter frame. It should be lined up directly with it. If not, then there is a positioning problem (timing) that has a number of causes. First diagnosis would be that the knotter drive is out of time relative to the trip dog.
If its not lined up, then make it line up by (they should then be parallel) by turning by hand, one of the knotter gears of the needle pickup link. This should put you in home position, with the knotter stop roller in the right intermittent gear notch. There should be 3 springs that assist the stop: 2 for the upper hay dogs and one for the stop mechanism.
Next: The clutch ring has a cam lobe on it that should now be in the 12 o'clock position. (That means straight up). You said it had been timed, so I expect this to be true. The high point (actually 'low' point) of this lobe is my timing reference.
Now look at the position of the set screw on the trip dog bracket (its the sort of round casting inside the clutch ring). The set screw should be positioned in between 2 and 3 o'clock as seen from the left side. I don't mean it can be anywhere between 2 and 3 oclock, I mean its got to be 1/2 way between these two angular readings. Normally this set screw rides in a dimple in the knotter drive shaft, but my first guess would be that its come loose, broken or the shaft has fractures or worn in this area and the trip dog has lost its angular reference zero. Yeah it may have caught on somewhere but its now out of position and the cam ends its travel without the drive shaft being in home position. While you are at it, peek inside the trip dog and make sure the engagement spring is there and its surrounding area is not all full of chaff, mud, stone or other fragments. Such stuff could confuse the clutch into thinking its done, when its not. The clutch ring has a steel follower on it. Make sure that its still there.
OK try all that stuff. If the set screw is bad of loose or out of position, reset it and be sure to tighten the lock nut.
Last possibility is that the knotter drive shaft is bent (twisted) as a result of a major needle drive collision. There is supposed to be a shear pin in the needle lift link to prevent this, but you can guess how that can go haywire (so to speak) !. If the trip dog set screw is in position and the set screw is on the shaft dimple but the intermittent gears and needle lift are out of position, then the shaft is twisted. Yes people have been know to straighten it by putting a large pipe wrench on the needle lift arm (remove the safety cover if necessary), and put a long pipe on the wrench (as in 6' extension). Its best to put 2 wrenches on it (one pulling up and one pushing down) to keep from cracking a knotter frame. Anyways, you might have to do this just to remove the shaft from the knotter cage because the twisted section will complicate the removal of the gears and the clutch ring.
I sure hope this helps. See if you can watch my Youtube video to check the sequence of events and positions when the knotter is tripped.
I'm really focusing on the clutch ring and trip dog action until you certify that its all timed correctly. Hope its not a twisted shaft. PITA.
