Originally Posted by Tinman
I think that marathon has a brand new sterling, these come with very light hammer and valve springs. It is a completely different animal than a proline sterling, with adjustable velocity and all. for consistency arrow recommends 600-650 psi hpa
If he goes and drops a cut spider spring in it, it will do nothing positive for his marker or his pumpstroke.
Then his gun has changed little to none from my AP stp from 2000. It's why I suggested turning the pump rod (if able) otherwise you might have to adjust the pick on the end of the pump rod which either has set screw or is loctited in. I suggested moving the pump rod as it is the easiest to undo if you make something worse. Most likely he needs to fiddle with the input pressure and get his adjustable hammer tuned properly.