I had a very similar problem with a friend’s 1962 40Hp Evinrude. It would start cold but any attempts to start it after that were to no avail, even putting in a new set of spark plugs. We swapped, mags, ignition modules, used starting fluid, etc. and the results were never consistent. There was no reason this engine would not re-start with healthy compression, good spark, air and fuel. Then on a test run where we shut it done and it would not start I made an observation that jogged my memory.
These engines used a Champion J6J or J4J. This was a special spark plug that had the side electrode cut back so it ended directly over the middle of the center electrode. This arrangement allows for a better spark due to the sharp edge of the side electrode directly over the center electrode to blast through oil and gas film that accumulates on the spark plug.
Since the Champ J6J and J4J is not available anymore the NGK equivalent matches heat range only, NOT the side electrode modification. We pulled the NGK plugs out, cut back the side electrode like the J6J had and ventured out again. NO matter when we shut the motor off, it always started within 1 spin of the flywheel, hot or cold, 10 out of 10 times without fail. It was like magic.
This weekend my friend took his boat out in the bay to do some crabbing, not a good place to have a motor that refuses to start as the sun goes down and wind whipped waves appear. His motor started every time, all day long.
If you aren’t using the Champ J plugs I suggest you modify whatever brand you are using so the side electrode ends over the middle of the center electrode. There is a reason the people at OMC and Champion made this spark plug unique in that aspect and I am not smart enough to fully understand it, I am just reporting this was the magic bullet for my friend’s 1962 40Hp Evinrude.
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