My 40HP Mercury outboard motor revs at very high rpm when in neutral. When advancing or reversing it returns to normal but when placed back in neutral it revs again. Any ideas as to the problem?
So with the throttle closed, held closed by hand, does it still over-revs as soon as you start it up? If that's the case, disconnect the top spark plug wire and restart it, if it doesn't over rev, the issue is the upper crankshaft seal. This is a fairly difficult job so I would suggest seeking out a marine mechanic unless you are very mechanically inclined.
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So with the throttle closed, held closed by hand, does it still over-revs as soon as you start it up? If that's the case, disconnect the top spark plug wire and restart it, if it doesn't over rev, the issue is the upper crankshaft seal. This is a fairly difficult job so I would suggest seeking out a marine mechanic unless you are very mechanically inclined.