High idle no load speed for the engine should be 1750. Since the PTO is a 3:1 reduction this would be about 585 to say 600 on the PTO shaft. When you lug it down to say 1400 RPM how much exhaust do you have. Also have you checked pump timing. I used to advance some of them one set of holes on the drive gear. You need a dynometer to tell you where you are at and then to tell you what each adjustment does. I was at a show in Indiana a few years and the man had a nice 560 Cockshutt on the dyno and it was at about half horsepower. Most of his problem was wear in the linkage.