Walt, this is a 1610D, 4WD and a loader. It has a round knob under the seat which I think is the open/close valve. It turns hard, and doesn't feel "right". I think its not opening all the way, because the little servo shaft at the back moves a little as you lower the 3 point lever, but quits moving too soon- so I think its still partially blocked out, or the shaft is hung up.
According to one of the rare pieces of actual information in the operators manual, the three PTO speeds at 2200 engine RPM are 530, 8 something, and about 1100. I think I'm in high- that would really make that bush hog sing, at least for the brief period before it blew up!
I never do anything any more complicated than an oil change anymore, without a manual. That practice came to me suddenly, and quite by chance- when I removed an innocent looking bolt on an old tractor, and instead of just falling into my hand when I got it unscrewed, the bolt, and the spring and checkball behind it, shot across my shop. Took about an hour to find all three pieces (my shop won't win any prizes for neatness), and I vowed, "Never Again!".