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Bought a 2000 Johnson 50 hp outboard one year ago. Just attached it to my boat and started it up. Motor idled and ran well for a few seconds, then took off on it's own up to a high RPM even with the throttle at idle. Motor didn't shut down when I took the key out, so I pulled both spark plug wires. It continued to run despite having no spark. I disconnected the fuel line which finally stopped it after about a minute.

What are some possible causes of this behavior?

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  • I'm no expert at all, but sounds like a spark plug problem to me. Try removing them and giving a clean to remove carbon. COuld also be overheating - you were running it in the water???
    – bob1
    Commented Jul 30 at 20:23
  • Spark plugs are pretty clean, some oil residue but no carbon build up. Yes, I was running it in the water. It started running away about 10 seconds after a cold-start, so I don't think it's an overheating issue. I'm starting to suspect that the oil injection system is over-oiling the mixture, causing it to diesel.
    – Carlton
    Commented Jul 30 at 21:56
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    @PMF: We're not talking about diesel. This is about dieseling, the ignition of petrol without a spark. I've seen it happen a few times if you put 4* in a car designed for 2*.
    – Chenmunka
    Commented Jul 31 at 8:40
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    It's a 2-stroke engine, and it has an automatic oil system (VRO) that mixes the gas and oil on-demand from a remote oil reservoir. I think I used 89 octane fuel in it.
    – Carlton
    Commented Jul 31 at 15:16
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    @Chenmunka - Your description of high octane and dieseling is inaccurate. Higher octane fuel will actually make it harder for dieseling to occur. That's because as octane increases, it becomes harder to ignite the fuel. Octane does not infer any more energy in the fuel, therefore it doesn't cause it to burn hotter. Only two things will cause it to burn hotter, that being the compression ratio and the fuel mixture (leaner burns hotter).
    – Pᴀᴜʟsᴛᴇʀ2 ♦
    Commented Aug 1 at 10:07

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