I say mumbles is correct—as far as it goes. The words for the situation probably are not correct. I have spent lots of time thinking and pondering over the past 60 years about exactly what is happening in an outboard motor in so-called “runaway” condition.
First, in an outboard the term generally means what happens when a motor is revved up in neutral or no-load situation, and cutting the throttle back does not slow it down. As suggested, it is not true thermal “runaway” It happens often in OMC Big-Twin and RD series motors and the sound is unmistakable when it is happening. The vacuum cut-out switch senses that condition and kills the spark on one cylinder till it slows down.
So what is causing it to run away? In my theory and opinion, it is flywheel momentum and manifold vacuum. Those motors have large and heavy flywheels for smooth running. Get the thing spinning fast and close the throttle, and abnormally high vacuum is created in the intake manifold.
Now consider the carburetor. It has the main nozzle and a large venturi. Great for high speed and lots of air flow. But slow it down and there is not enough air flow through the venturi to make it work. So a second fuel and air flow route is provided for the idle and intermediate speeds. You could compare it to a second, smaller carb. Briefly, it consists of a hole or holes in the throttle butterfly with holes in the carb body critically placed in relation to the butterfly in closed or cracked-open position. At idle, fuel and air flow through these holes are what is feeding the cylinders. The main venturi and nozzle really aren’t doing anything because of the slow air flow through that area.
So…now you create an abnormally high vacuum in the manifold. That high vacuum sucks more air in through the carburetor. Enough more air to also draw in more fuel, possibly even some from the main nozzle. Bottom line is this extra air and fuel is enough to provide the “kick” that is keeping the flywheel spinning rapidly. Killing the spark prevents the extra fuel/air from igniting, and it slows down
BTW, I also have had a couple of hair raising experiences. One was a 40hp Scott-Atwater (no vacuum cut-out switch). I had to pick up a tool and bust off a spark plug to slow it down. Worse, was a Diesel that I accidently got into true runaway. I was so panicked that I don’t truly remember how I slowed that thing down. Suffice to say to me that I shouldn’t be working on Diesels.
That’s my story and I’m sticking with it. At least till somebody corrects me.
