Runaway motor with plug leads removed

[legendre]

@johnnyrude

As others have illustrated, it’s a very real phenomenon, and there’s nothing magical about it – it’s dieseling, simple as that. The cause is generally combustion chamber build-up which increases chamber pressure and creates localized hot spots that act as igniters for the overly compressed charge. It can also be prompted by spark plugs of too high a heat range, which may get hot enough to glow.

Low octane fuels are another contributor. Remember that fresh gasoline can lose +around+ one octane point per week, when in storage. This is just a general rule of thumb, though, and is highly dependent on storage conditions and initial fuel quality / composition.

[johnyrude200]

I wish I could see what you’re talking about with hard carbon build up, Pappy. I’m guessing nothing I’ve worked on comes close to that situation, but the motors I have seen so far can sometimes have ‘chocolate frosting’ or carbon on the pistons, cylinder head, and of course the exhaust side of the motor. I really only work on stuff up to 40hp at this time.

[legendre]

@johnnyrude

Four-stroke motor, in this case, but the issue is the same.. the piston is all coked up. While the cylinder head isn’t pictured, you can safely assume it’s about the same.

[johnyrude200]

Ive had some motors that caked up before. Just figured it was a motor running too cold, too rich, or with bad fuel…havent had any motors try to run on their own, but I do recall owning a 1976 chevy pickup that would stay running after I turned the ignition off for usually 15-20 seconds with spits and spudders. Who knows why, may have been something totally different. I was 15 at the time and just happy to have a set of wheels!

[brian-grigsby]

I had that happen on my 81 Evinrude 50hp. While on muffs running in my driveway. Idled it up slightly and it took off! Pulled spark plugs quickly and it kept going. Kill switch did nothing. Finially choked it and it died. Luckily no damage as she’s still running well to this day.

[legendre]

quote Brian Grigsby:

I had that happen on my 81 Evinrude 50hp. While on muffs running in my driveway. Idled it up slightly and it took off! Pulled spark plugs quickly and it kept going. Kill switch did nothing. Finially choked it and it died. Luckily no damage as she’s still running well to this day.

Did you ever open it up and determine the cause?

[green-thumbs]

Actually electrical ignition systems were not used on early gas engines…they had something called the hot bulb also there were a type of semi diesel engine that required use of a blow torch to preheat a spike or plug that stuck into combustion chamber. Needless to say magneto
or Delco system or modern solid state ignition are superior in many respects but once upon a
time engines did not have spark plugs or electrical systems. There were also electrical ignition systems with points inside cylinder. As weird as some early ignition alternative were they all
worked well enough to sell and satisfy customer until something better came along.
Remember the cars that would run on a few decades back when Detroit was trying to clean
up emissions and increase fuel economy? You had to leave it in gear when turning off the key
to get them to stop.
Louis

[brian-grigsby]

quote legendre:

quote Brian Grigsby:

I had that happen on my 81 Evinrude 50hp. While on muffs running in my driveway. Idled it up slightly and it took off! Pulled spark plugs quickly and it kept going. Kill switch did nothing. Finially choked it and it died. Luckily no damage as she’s still running well to this day.

Did you ever open it up and determine the cause?

I did not. It still runs fine to this day. I just don’t ever rev it on the muffs. Learned my lesson. In the lake you can rev it and it’s fine. Must be the lack of back pressure while on the muffs.

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