So what are you going to do with it now? I think that if it was mine, I would readjust the exhaust valves, with a couple of thousandths extra clearance, and then see what a compression test would show. If you are a little lucky, the engine will run fine, and you can put it on your list to check the valve clearance more often in the future. If you are less lucky, you might have burned exhaust valves, which might be salvagable after you tear it down, or which might need to be replaced. If you end up having to remachine the head, I bet there is a shop somewhere that can install hardened seats. I would also look into the possibility of getting stainless steel exhaust valves.
I have read about exhaust valve seat erosion and recession for years, but in my personal experience, it never has seemed to be a problem. Especially on old tractors, which seemed to have been built to stand almost anything, and which were seldom worked that hard. But apparently your JD is having a problem that is losing your valve clearance and that has to either be erosion at the valve and seat area, or the other possibility is that the valve stems are stretching. I have seen valve stem stretch in very hot running, air cooled small engines, but never in a liquid cooled engine. As a matter of habit, I always add a thousandth or two of clearance from the specs for valve clearance on solid lifter engines. I would much rather have a little bit of valve noise than too little clearance that results in burned valves. But that is just my way of doing things...and maybe I am wrong.
I sure think tinkering with it a little would be worth a try, and doing so would not cost almost anything except some time. Good luck!
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