Reply To: 1959 Evinrude Lark 35 hp tear down

[Don]

Well, could have been so many things….Doesn’t look like the block was overheated though, not discolored. You might have a look at the plug threads, perhaps someone put some thread insert in there that might have allowed the plug to heat up too much. What size prop is on the engine? Perhaps someone had it overpropped and was lugging the engine at high RPMs. Not enough oil/lousy oil is always a possibility… Perhaps someone had some hot plugs in there trying to make it idle better. The engine is in such nice shape otherwise, looks like the gearcase was replaced though, surely was only used in fresh water.
Perhaps the engine was partially overheated, worn pump, no thermostat/poppet valve.
Check the mag plate stops to make sure nothing is broken that might be allowing it to over advance.
Finally, keep in mind that those compression relief valves might be messed up causing the low reading in one cylinder, even though the cylinder is clearly scored, and there is aluminum transfer…Kinda looks like detonation/preignition damage, but who knows….