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Run Away Engine!

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Porterhouse

New member
Was running a 1987 50hp evinrude engine today. Noticed it idlein rough then all of the sudden it starting reving up on its on like to the moon :). So i rushed around to try to find some pliers to pull wires off. I found some an pulled wires off an it continued to run. By then my technician was freaking out he didnt know what was going on lol. I finally put a rag in the intake an it finally shut down. But what an exciting morning to say the least. So my question is what causes a run away engine I have always heard about them but havent ever run into this. thanks
 
I have never seen it happen but I believe what happens is that if you over-rev the motor in neutral it can heat up the combustion chamber to the point that the spark plugs will pre-ignite from the heat alone. In other words, even when the voltage is removed from the plugs the spark keeps happening and hence you get a run away engine. It will continue until the fuel in the carbs is all gone or till your tank is empty if you haven't removed the fuel line yet. Good move on the air suffication strategy. If left on its own it would most likely eventually throw a rod.
 
Might not be the plugs glowing in the dark, but carbon deposits it the combustion chamber and piston tops. Do a de-carb.
 
haffiman37 said:
Might not be the plugs glowing in the dark, but carbon deposits it the combustion chamber and piston tops. Do a de-carb.
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Yes. That does make more sense. I really can't see a spark plug sparking if I threw it into a campfire. If my motor did this I would want to do a compression test to see how my head gasket was holding up and then I would de-carb the daylights out of it.
 
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