C&P Jumping on impression

Our C&P has moments (not always, it goes through stages) whereby when we make an impression the press jumps twice, makes an awlful noise and is no doubt causing problems to our beloved press. Adding or removing packing doesnt seem to make any difference (it does it with no packing, and also with our maximum level of packing). Our platen is nice and even, and we have no other printing issues.

We seem to be able to fix the problem by getting the flywheel into the right position and hammering in the pulley key. It fixes the problem, but it always inevitably seems to come back eventually. Any thoughts on whats causing the problem, or permanant fixes we can try?

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possibly check that the gate lock is not interfering with the rocker. it would make a clunk on closing the press, but if way out of time,or the roller is badly worn, would make a solid bang or thump when opening the impression. your platen casting is bolted to the rocker casting with the impression screws. the rocker is the part that actually turns in the press frame. when standing at operator side look down to the right as you turn the press over you will see the rocker bearers come up to meet the press frame. the gate lock has 2 bearer surfaces that engage under the rocker just on impression to lock it in place. there should be no premature or overly extended contact between the two. i can elaborate more if this is the problem. Eric