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Message: <blockquote data-quote="C&amp;amp;K" data-source="post: 482214" data-attributes="member: 30707"><p>I have seen several Mercury 40's with bad center seal on the crank. The four-cylinder ones tend to break the aluminum seal carrier on the center main bearing. The twins like in this thread, the split seal ring on the bottom of the center main bearing goes bad with time, especially if the engine sat for a time without being properly winterized and the crankcase dried out. When that center seal goes bad they will still run but won't idle no matter how you tune it. The fours will tend to drop the two center cylinders at idle. The twins will run rough if the idle mixture is set rich enough. But the fix is to split the cases and replace the seals. While you got it apart may as well replace the lip seals on the crank ends too. I've done probably a dozen of 'em over the years.</p></blockquote><p></p>