As mentioned, you need the sickles to be registered with the guards. A proper pitman length should be right. There are shims to add or subtract near where the bar pivots, on the three bolts, to make minor adjustments.
Itís important, but it should naturally be right nearly all the time.
I got a disc mower 5 years ago, pulled my 501 out this fall for trimming under a fence wire, took a little greasing but it shined up.