old Gravely stops running when hot

[flagpond]

I have an old pre-1969 Gravely 10 HP commeriial walk behind with a 1 cyl Kohler engine. it is used as a tiller. It runs fine until the PTO is engaged. Then, after tilling 2-3 rows, it starts sputtering and bogs down and basically quits tilling. Sometimes it simply dies. I unengage the PTO and sometimes I can get it to start running normal again but usually it sits there struggling. A little black smoke is emmitted. Then, when it cools down, I can again till 2-3 rows then it does it again. Does anyone have a theory as to what is wrong?