Ok, James I want to be sure we are talking about the same thing, there are 2 rods on the pump, one is the Pickup Rod the other is the Pump arm. right pickup rod, I mentioned in an earlier post this thicker rod, but couldn't remember the proper term so the pickup rod is the one in question. Really though it should be called the pump rod as that is what it actually does. The socalled pump rod only moves the bolt back and forth, which isn't connected to anything else. oh well. pickup rod it is then since that is probably what the manual guys went with.
The pickup rod has two different thicknesses with a step in the middle of the shaft the thicker end being in the pump handle. because of the step in the middle of the shaft there is no benefit to changing the depth of this rod in the pump handle. The pickup rod will always enter the marker the same distance stopping at the step. This is where the stop nut (pickup collar)comes into play (large brass nut that the pickup arm slides through) by adjusting this nut you control the depth the pickup rod enters the marker. true. I forgot about the step, though it seems I have adjusted pickup rod this way once or twice, not sure why it would have worked given your explanation. The brass nut sounds like a better plan. I didn't know it was adjustable. I'll check. Internally, the pickup itself can be adjusted and could be the cause of the op's trouble.
Note: the brass collar on my stp is round on the outside(is it adjustable inside or from the outside on yours?). I believe my old proline bronze was square or hex shaped on the outside.
When you change the pump arm position it only changes the position of the bolt.
unless you have it wound too far in and it impedes the pickup arm from being able to return all the way forwards. (causing trigger engagement issues) true, my bad for using the wrong term.
In a pumping motion, The pickup on the bottom of the pickup rod compresses the spring and picks up the hammer and pulls it to the forwards position. ( If the pickup is too far forwards on the rod it may not grab the hammer, if it is wound too far out it will pickup the hammer but may not engage the sear properly ) I also wonder if the pickup is to far forward if it would put excessive pressure and open the valve a little?
I really don't want to argue the point that Springs have changed..But if i must, Hammers with the adjustable velocity nut are intended to have springs that are significantly lighter and shorter than proline springs Arrow has them listed as STP springs.
I ordered a handfull from Arrow for my Proline stp, it didn't work out. I cut spyder springs got it back to where i wanted it and later sold it.
I converted a 2000 stp by buying a adjustable hammer, a main spring, and a valve spring. I bought and installed a pickup assembly as well but the pickup assembly was not necessary but the stp pickup assembly is hollow and allows for external velocity adjustment.
I also snagged a rt feed spyder threaded sterling with current stp adjustable internals
Everything i have now runs the newer hammers and springs.