Runaway motor with plug leads removed

[johnyrude200]

A couple of years back I read some posts about a phenomenom where a motor can kick over and run with the plug leads off. At the time, I didnt think anything of the post, but now that Im full time on these and safety conscious due to the volume I do each week, I was wondering if there’s any truth to this?

[mr-asa]

Maybe, but I really doubt it. To do so it would have to be dieseling. If the engine is hot enough, and has a high amount of carbon build-up that might create a hot-spot that would allow the mixture to light off. But I’m not even sure you’d get those conditions in one of these motors. It can happen with carbed gasoline 4-strokes though.

I’m fairly sure you’d have to have it running beforehand, though.

[frankr]

Actually. some motors are very prone to doing that. But it is runaway of a running motor, not one spontaneously starting up not running with the plug leads off. So I think you are mistakenly combining a truth with a not-true.

EDIT: OR perhaps you have heard about the early V-4s with battery CD ignition that could start and run by simply turning the key on but not running the starter. Sometimes they would even run backwards when that happened. They had to come up with a safety switch that would not allow spark from the ignition until the starter was actually turning the flywheel.

[frankr]

quote FrankR:

Actually. some motors are very prone to doing that. But it is runaway of a running motor, not one spontaneously starting up not running with the plug leads off. So I think you are mistakenly combining a truth with a not-true.

EDIT: OR perhaps you have heard about the early V-4s with battery CD ignition that could start and run by simply turning the key on but not running the starter. Sometimes they would even run backwards when that happened. They had to come up with a safety switch that would not allow spark from the ignition until the starter was actually turning the flywheel.

Even those had to have the plug wires attached for that to happen

[chris-p]

It absolutely can and does happen!

Turning the key off wont stop it. Nor will pulling the spark plug leads. Nor will pulling the fuel line, as it will run on the fuel in the float bowl. The only way to stop it is to take a rag, and cover the intake of the carb and seal it off.

What causes it is carbon build up in the cylinder. That, and a lack of backpressure on the exhaust while running. Happens while running on muffs mostly, on bigger HP motors. Small hp motors are not as prone to it. I have a large test tank, so I have never encountered it, but my neighbour that loved to rev up his big old girl in the drive way on muffs threw a piston one Saturday afternoon after I had warned him against that practise many times.

Moral of the story, don’t rev up motors without sufficient backpressure on the exhaust, and keep them carbon free. I do annual de carbs with seafoam, or OMC engine tuner. Do at night, or your neighbours will think your house is on fire!

[Tubs]

A "Boathouse Repair" is one that done without having tools or the skills to do it properly.

• This reply was modified 6 years, 6 months ago by Tubs.
• This reply was modified 4 years, 2 months ago by Tubs.

[johnyrude200]

I was surmising a motor cant run without the plug leads on for obvious reasons, but It’s always better to ask first, then lose an appendage and talk about it later.

On a side note, has anyone heard of a crank breaking off at the taper and the flywheel letting loose?

[frankr]

1960-61 Evinrude and Johnson 40hp motors have that reputation. That’s why they made the crank bigger starting in 1962. I hasten to add that obviously not all of them broke, as witnessed by so many out there still running.

[chris-p]

Tubs that bike looks mint!

[Casey Lynn]

Years ago I was sent in to visit and diagnose one of these engines. Probably late 70’s or very early 80’s.
The dealer was Vero Marine Center and the mechanic/service manager was Tom Council
The engine was a V6 crossflow 235hp.
Once the engine was run hard (we had a test wheel so we could duplicate it at the dock) the key could be shut off and the engine would continue to run.
Verified no spark at the plugs. Ignition shut off had grounded the packs as designed. Pulling plug wires made zero difference.
The owner of the engine had been running nothing but air-cooled two-stroke oil in the engine since new. He owned a small engine shop that sold lots of air-cooled lawn equip, etc.
Once the cylinder heads were pulled the amount of hard carbon build up was amazing. The engine dieseled on this pretty easily.
Once cleaned up the engine ran pretty good except for stuck rings. Repeated engine tuner applications were needed to free them.
Don’t remember how it all turned out except that the customer was raising Hell because I would not cover the repair.

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