Massey-Ferguson Baler

As far as I am aware the timing is essentially the same as for the later '20'.
First of all empty the chamber of hay and cut the cord from the knotters. Turn the flywheel by hand in direction of rotation (top towards pick up) Engage the knotters by the star wheel. Move the plunger slowly down the chamber. The needle tips should just start to come through the floor of the chamber. At this point the lower tips of the plunger face should be just past the needle tips (1/2" to 1 1/4") so that the needles come up in the plunger slots. The most common cause of needle failure is when the Knotter brake (see below) has failed and a mass of hay is pushed down the chamber with the needles already in the chamber. At the point where the plunger knife is fully past the stationary knife the packer arm should be at it's fullest lift before returning to push more hay in.
If the needles remain in the chamber as the plunger comes down you will need to manually pull the needle yoke back to its rest position, this will reset the plunger 'dog'. There are a pair of quadrant brake pads on the needle yoke. When these wear, spring pressure is lost and the yoke can fall under its own weight or by 'bouncing' across a field, causing the needles to enter the chamber as the plunger is coming down. Hence Big Bang! They can enter either before or after the plunger passes the plunger dog.
I think this will be your problem. Either the disc is rusty (if it worked ok before, it should be clean), not enough spring pressure or the pads are worn down to the rivets. Take both pads off and check them. Don't risk tightening the bolts only as if they are worn it could be an expensive mistake. Pads can be relined at any shop that relines brake shoes etc at a fraction of the price. With regard to the packers staying in the up position; This normally happens when the plunger has come to rest on the charging stroke. Besides looking ugly it will be difficult to restart the baler as the initial movement of the plunger will be to compress the hay in the chamber with no momentum to follow it through. With our '20' I always turned the flywheel backwards to bring the packers down and give it some momentum for its next start. When you get to the point of everything working, re-thread the needles, have someone hold the cords on the chamber as if it is a small bale and turn through the knotter cycle. The yoke should return to rest and hopefully you will have two tied cords. Hope this helps. Please come back if you need any more help.
Regards
DavidP