The end of the story on the Honda is that rust had taken its toll on the clutch.
The whole design of that clutch is really brilliant. When the clutch is supposed to be on, there is a spring that holds the two halves of the mower drive pulley tightly onto the rotating friction disc. When you turn it off, another brake assembly moves forward and punches that spring loose and at the same time stops the blades by using a brake pad onto the pulley.
I love it. It engages smooth unlike an electric clutch and also stops the deck nearly instantly.
My clutch had three main problems. One was that the brake pads were worn down to nothing, which probably was a symptom and not really a cause. Two was that the fiber lining on the drive disc had come loose just due to corrosion, making that disc about 1/4 inch wider than it should be. Three was that the metal linings that grab that drive disc had rusted to the fiber disc and had also been pitted.
I replaced the brake, clutch disc, and both metal linings. Once I adjusted the clutch, that cured it.
I see now why people love these tractors. It starts the instant the starter touches the flywheel, it will run all week on one tank of fuel, and it mows like a dream.
Makes me wonder why Honda ever quit making them!