Could be a valve sticking when the engine gets hot.
I had a 1000 that acted the same way. When the tractor was cooling down I could hear the valve pop back into place. I could start back up and run for a little bit after that.
Something to try that is very inexpensive to get a bottle of Marvel Mystery Oil. Add a heavy dose to the gas and the oil.
I did this on my 1000 and after about 2 tanks of gas, this problem went away. I mowed once a week with that tractor for another 8 years before I sold it and never had this issue again. From then on I always add a little bit to the gas and oil in all my Cubs. Saved me from tearing into the engine and doing a valve job.
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