Check the valve clearance... Mine did that and after trying everything else it turned out to be a valve adjustment back to specs cured it... it would not even turn over... when it cooled the compression dropped off and the starter could do its job... It seems that the thing has an automatic de-compressor built in that allows easy cranking rotation if the valve gap/timing is right on... Wish I knew how to explain it all and the details but I'm sure someone here will know enough to explain better...