My 1983 317 is shutting down with the PTO engaged. Pretty quickly, I'll engage it, mow for a minute or two and then it will shut down. A couple times I engaged the Pto at a stand still and when I went to drive forward it stalled. I'm pretty sure the seat safety switch was completely removed or bypassed by the previous owner who did a full strip down and restoration on it. I've had no issue jumping off the tractor to pick up a stray dog toy in the lawn while mowing in the past.
Here are the tests I have done to trouble shoot. In the garage I removed the deck belt and engaged the pto. I throttled up and down and hoped off the tractor. It ran fine. I checked all the pulleys and everything felt free and smooth enough. I checked all the wire connections behind the dash as best I could and couldn't find anything obvious.
I also drove the tractor around with the PTO off, throttling up and down on hills in attempt to add load the way the PTO would when engaged. It felt fine.
I have owned the tractor for a month and it's been pretty flawless until now. I've been mowing, towing and driving around without any issues. A few days ago I gave it a really good wash. I even covered the intake and sprayed the engine down lightly. This was the first mow since the wash, but I have been towing with it.
I'm stumped for now, and every search I've done says check the switch and belt path. I don't think the carb is "perfect", it could probably use a little cleaning and tuning, but it's not that bad, it doesn't hunt, but it doesn't throttle up as smoothly as my other machines. It takes a second to steady out the rpm as you move the throttle. 3200 seems be the sweet spot, above that it is less steady. It's a Kohler Magnum 18, M18QS.