One more “final update”. Yeah I know I said the last one was the last one, but this really is. So after some more messing with it, I just didn’t believe my own BS, so I checked the tach. I put a photo tach on the crankshaft pulley. Little piece of reflective tape on the nut, actually, and was able to get a stable read. As the mechanical tach inidcated 1500, the engine was actually turning closer to 2500. Indicated 1000 was closer to 1800, and indicated 500 (idle) was closer to 900. So, indicated 1500 was actually pretty close to redline, and I shouldn’t have screwed with it AT ALL! Ugh. I did manage to learn a lot about these pumps, so not a total loss, but a giant PITA, to be sure.