Does your JD by any chance have an OHV Briggs and Stratton V-twin engine? The problem you have described is typical of what happens if the tappets are out of adjustment: the decompressor then does not work, and the engine is excessively difficult to turn over. This results in a lot of unnecessary replacement of batteries and starter motors, and burned insulation on wiring.
If I recall correctly it is the inlet tappet that is critical - it appears that as little as .001" too much clearance will cause the decompressor not to work.
